I made the joke about the Cowboy Cadilliac because those had 2.7x gearing and even with a 472 or 500 big block would get mileage in the 20's on the highway.
The things were heavy pigs and even with 500ft/lbs of torque were slow as heck acceleration wise. This was mainly due to the gearing. But due to the gearing the engines also were generally run at lower rpm and could reach 300k mileage when other engines of that time were lucky to reach 150k.
I know with a sm465 I can beat the heck out of the thing. Do things like slap it into 3rd at 70mph to pass someone or stuff a 500 ft/lb engine in front of it with no transmission modification. The thing will run with bad bearings, bad seals, low oil, and once back at the house can be rebuilt in a weekend without specially tools.
The 420 is the 465's father and is the nv4500's grandfather. The 3 transmissions share very similar parts and are tough. I consider the 4500 less reliable then the 465 due to the overdrive issues.
I did bolt a sm465 into a rusted 76 Cadilliac back in 98. It was not pretty though. There was a saw all involved with the floor, manual clutch linkage, and the driveshaft extended with a rusted pipe. I was young, broke, and the car was cheap.
The thing was slow. But once I got over 45mph I could just cruise in 3rd using that big blocks torque to pull the car without needing to shift much. Could drop it down to 30mph in 3rd lugging that girl back up to 70 before shifting. Once I got up to highway speeds the thing would cruise. Steering and brakes were on par with a typical land yacht. But the engine was happy going 85mph with the 2.7x gearing.
On the other end of the gearing scale. I had a 84 13,000 lb c30with a 8x12 flatbed. The thing had a smog 350, sm465, and a FF 14 bolt with 4.56 gearing turning 235 tires. I used it for towing and tearing apart forklift trucks for parts/scrap. The things bed was high enough a fork truck could be loaded right off a semi dock in many cases and I cut them up right on the bed to get the electric motors, controlers, etc.
That 350 was only able to move with around 9,000 lbs (somewhat safely) because of that 4.56 gearing. I avoided the highway generally and drove it at 60. I eventually dropped a 383 into the thing around 350hp and was having fun (unloaded at a mere 6,000 with the speedometer needle buried most of the time on the highway getting mileage around lets say 6mpg. Eventually something had to go and in my case it was the rear main at what I would guess was 5000. The engine just locked, shut off, and the clutch slipped before I pulled it over. I bought some 80/90 and poured it in. Drove it to a rest area then drove the thing to the folks house where due to limited space to store the truck at the time I cut it up for parts. I have a video of driving it with the rods threating to come out the side of the block.
Over 10 years later
I bought my recent 89 3500(squarebody) with tbi 350, sm465, 4.56 14 bolt with 245 tires with the hopes of doing the same things I did with the 84.
For what your doing I would look for a 5 lug 10 bolt with a 3.42 gearing from a 90's 1500. Thats popular enough you should be able to swap axles rather easily/cheap(you may need to fab some brackets).
If this is truck is a old 6 bolt 1/2 ton then an adapter can be used to keep the 6 bolt look or you can run 5 bolt hubs on the front and then run 5bolt rims all around.
Your going to loose a little mileage. But I have ran 3.42 gearing with a 465 and its not that bad. The torque diffrence will be noticable on hills and when you need to pass. You should still be able to slap it into 3rd on the highway and bring it up to 75 to get around a semi before going back into 4th and cruise at 70.
