Cousin_Joe said:
I'm looking at my summer daily driver. It's a '70 Impala (~4200 lbs). I currently have a PG trans, and 2.73's. I'm really considering replacing it with a built up 200-4R for the O/D benefits (I'd probably be changing the rear to something like 3.36-3.55 if I do go this way). We'll say 99% street, economy is a concern as I put on a ton of miles (and I've never been to a strip before). I'd just like to unleash a little of the 350's fury once in a while down the road, and have an engine to be proud of.
I'll hit the transmission first, I think a 700R4/4L60 would be a better choice for a gear box for a couple reasons: First, it's built to react more torque than a 200R4 which is more appropriate for a 350 on one end and 4200 pounds of Impala on the other; Secondly, it has a deeper low gear than the 200R4 which will help launch the 2 plus tons on Impala while still giving a decent low speed over drive in 4th.
A lot of your dissatisfaction with the current 350 can be attributed to the 2.73 gears, the 2 speed Power Glide, and of course those 4200 pounds. Now I'm not saying not to mess with the motor, but! The "but" is that the gears and gear box and the weight have all been working against this motor for all these years so if I had a budget of 1000 bucks, I'd start under the car rather than under the hood.
Now that said, it does appear that you need a valve job and that got you thinking about heads. That's really appropriate given this engine been running with unleaded fuel for 25 years and was never designed to do so, therefore, it's likely a lot of exhaust seat regression has set in and restoration would include hard inserts which will add to the cost of rebuilding. So considering replacement heads with hard seats is reasonable as the cost of rebuilding will be close to that of replacement. Here the 060 original Vortec looks cost effective except it needs a unique intake bolt pattern so a couple 3 hundred bucks for the intake has to be figured in. The latter BowTie Vortecs 421 and 431 are attractive in that they'll accept your current intake but they are more costly than the 060 which might be a wash against the intake replacement cost. Here's where I like the new S/R Torquers as they have the nice Ricardo, heart/kidney shaped, chamber of the Vortec, you can pick your port size, in your case I try for 160-170 cc for bottom end torque improvement, the torque peak moves up the RPM curve as port size increases and the carb sees less signal which drives you to richer jets to compensate so gas mileage starts to decrease which is why I'd go with the smallest port volume I can get for this. The S/Rs can be had with your choice of intake bolt pattern, spark plug type gasket of bevel seal, spark plug angle or straight mounting, drilled for perimeter or center bolt covers, stuff like that. So against the GM parts you've got more configuration and installation flexibility, plus they're SMOG certified which the Vortecs are not as a replacement head. I'd go with the 43610-1 for a 170 cc port, 1.94/1.55 valves, 67 cc chamber to run the compression up a tad, 86 and older intake bolt pattern, straight plug. I have to say that I like the GM Vortec spark plug location and the deck that interferes with the short side flow into the exhaust valve a bit better than the S/R chamber, but that's getting into nit picking.
For the GM Vortec either the 060 or 421 would provide the smaller port size I think apporpiate for your application. The 060 will require a unique intake unless you feel capable or can get a qualified person to redrill/retap the heads for the older bolt pattern. The 421 will most likely accept your current intake, but visually check to make sure before starting the engine.
If you use the GM Vortec, the lack of exhaust heat under the intake will be a problem for you till the engine gets good an warm. Two ways to go at this are: First, to restore the snorkel heat of the original air cleaner; Second, to clean connect into the heat cross over of the intake with NTP fittings to plumb it into the heater/bypass circuit so coolant will flow thru here and warm the intake. These two things will eliminate cool/moist day icing under the throttle plates, speed engine warm up, and reduce low and cruise speed cylinder to cylinder mixture distribution problems making the engine run smoother and deliver better fuel mileage.
The down side to putting either a top end rebuild or a new set of heads on an old engine is that the resorted compression starts pulling oil around the old rings, so don't be surprised if this doesn't beget a bottom end rebuild as well. But like I said earlier, I'd start with the rear gears at least 3.08s then the tranny in that order.
Bogie