Originally Posted by cdminter59
If the truck and engine are both 1968 it is probably a 396 ci or 427 ci. Both are good engines.
So did they Ever put a 454 in a pickup from the factory??
This may help!
the 396 was actually only put in 68 and 69 trucks, after 1969 they didn't make any more 396 big blocks, they were all 402.
The 396-cubic-inch (6.5 L) V8 was introduced in the 1965 Corvette as the L78 option and in the Z16 Chevelle. It had a bore of 4.094 in (104.0 mm) and a stroke of 3.76 in (96 mm), and produced 375 hp (280 kW) and 415 lb·ft (563 N·m). This version of the 396 was equipped with four bolt main bearing caps and was very comfortable with being operated in the upper 6000 rpm range.
Introduced in 1970, the 402-cubic-inch (6.6 L) was a 396-cubic-inch bored out by 0.030 in (0.76 mm). Despite the fact that it was 6 cubic inches (98 cc) larger, Chevy continued marketing it under the popular "396" label in the smaller cars while at the same time labeling it "Turbo-Jet 400" in the full-size cars. The 402 label was used in Light Pickup Trucks.
The 366 Big block V-8 (6.0 L) gasoline engine was used only in Chevrolet Medium duty trucks and in school buses. It had a bore of 3.935" and a stroke of 3.760". This engine was made from the 1960s until the mid-1990s. The 366 used 4 compression rings on the pistons as it was designed from the very beginning as a truck engine. The 366 was only produced as a tall deck engine with a 0.400" taller deck than the 396, 402, & 454 short deck big blocks.
Mark IV engines saw extensive application in Chevrolet and GMC medium duty trucks, as well as in Blue Bird Corporation All American and TC/2000 transit buses (the latter up until 1995, using a purpose-built, carbureted 427). In addition to the 427, a 366-cubic inch (6.0 liter) version was produced for the commercial market. Both the 366 and 427 commercial versions were built with a raised deck, four bolt main bearing cap cylinder to accommodate an extra oil control ring on the pistons. Unfortunately, the raised deck design complicated the use of the block in racing applications, as standard intake manifolds required spacers for proper fit. Distributors with adjustable collars that allowed adjustments to the length of the distributor shaft also had to be used with 366 and 427 truck blocks.
Mark IV engines also found themselves widely used in power boats, a natural application for these robust power plants. Many of these engines were ordinary Chevrolet production models that were fitted with the necessary accessories and drive system to adapt them to marine propulsion. Mercury Marine, in particular, was a major user of the Mark IV in marine drives, and relabeled the engines with their corporate logo.
Chevrolet gave all 427 engines except the ZL1 a torque rating of 460 lb·ft (620
1966 1969 L36 4-barrel 10.25:1 390 hp (290 kW)
1966 1969 L72 4-barrel + solid-lifters, more aggressive cam and high flow cylinder heads 11.00:1 425 hp (317 kW)
1967 1969 L68 L36 with 3x2-barrel carbs. 10.25:1 400 hp (300 kW)
1967 1969 L71 L72 with 3X2 barrel carbs. 11.00:1 435 hp (324 kW)
1967 1969 L89 L71 + aluminum heads; RPO L89 also applied to L78 "375 HP" 396 engine with aluminum head option. 11.00:1 435 hp (324 kW)
1967 1969 L88 Racing-spec cam, high-flow aluminum heads (casting #s varied by model year) and some upgraded, competition-grade parts 12.50:1 430 hp (320 kW)
1969 1969 ZL1 Aluminum block with open chamber "3946074" aluminum heads; cam even "hotter" than L88's; upgraded parts similar to L88's 12.00:1 430 hp (320 kW)
1970 1977(?) ZLX L88-ZL1 hybrid; iron block with aluminum heads 12.25:1 430(?) hp (321 kW)
The big-block was expanded again for 1970 to 454.2 cubic inches (7.4 L) with a 4.251 in (108.0 mm) bore and 4 in (100 mm) stroke. The 1970 Chevy Corvette LS5 version of this engine produced 390 hp (291 kW) and 500 lb·ft (680 N·m), and the LS6 engine was rated at 450 hp (340 kW). It has been suggested that the LS6 was substantially underrated and actually produced well over 500 horsepower (370 kW) as delivered from the factory, although there is no empirical evidence to support this claim. Indeed, the AHRA ASA Class record holding Chevelle LS6 for the 1970 season posted a best of season trap speed of 106.76 mph (171.81 km/h) "1970 ASA LS6 454 Records", which suggests something on the order of 350 "as installed" (SAE Net) HP for a 3,900 pounds (1,800 kg) car and driver combination. Indeed, SUPER CHEVY MAGAZINE conducted a chassis dyno test of a well-documented, well tuned but production-line stock 1970 LS6 Chevelle and recorded 283 peak HP at the wheels  - a figure that lines up quite well with the previously referenced 350 SAE Net HP figure.
A 465 hp (347 kW) and 490 lb·ft (660 N·m) version of the 454, dubbed LS7 was also designed but never went to production. However, a handful of LS7 intake manifolds were produced and sold by a few Chevy dealers as performance parts. The LS7 was later offered as a crate engine from GM and advertised at 500 Gross HP.
Power began falling off after 1970, with the 1971 LS5 producing 365 hp (272 kW) and 465 lb·ft (630 N·m), and the LS6 option coming in at 425 hp (317 kW) and 475 lb·ft (644 N·m). Only the LS5 remained in 1972, when SAE net power ratings and the move towards emission compliance resulted in to 270 hp (200 kW) and 390 lb·ft (530 N·m). The 1973 LS4 produced 275 hp (205 kW) and 390 lb·ft (530 N·m), with 5 hp (3.7 kW) and 10 lb·ft (14 N·m) gone the following year. Hardened valve seats helped allow these engines to last much longer than the earlier versions, even without the protection previously provided by lead from fuel. 1974 was the last year of the 454 in the Corvette though the Chevelle offered it in the first 1/2 of the 1975 model year. It was also available in the full size Impala/Caprice until model year 1976.
GM continued to use the 7.4 L (454 cu in) in their truck line, introducing a new Vortec 7400 version in 1996. GM also introduced the 7.4 L 454 EFI in 1987 (GEN IV 1965-1990, GEN V 1990-1995, and GEN VI in 1996); the GEN prefix was used since Ford Motor Company owns the Mark V naming rights since it was used on a Lincoln automobile between 1977–79), which was electronically fuel injected giving more power and torque. The 454 EFI version was rated from 230 hp (170 kW) to 255 hp (190 kW) and from 385 lb·ft (522 N·m) to 405 lb·ft (549 N·m) of torque. The 7.4 L 454 EFI was found on GM 2500 and 3500 trucks made in 1987, until replaced with the Vortec 7400 (GEN VI) in 1996.
1970–1976 Chevrolet Caprice
1970–1975 Chevrolet Chevelle
1970–1975 Chevrolet Monte Carlo
1970–1975 Chevrolet El Camino
1971–1972 GMC Sprint
1970–1974 Chevrolet Corvette