You can try it to see how it works, but what you will find is that on level ground, it might cruise okay, but you will need to really crowd the throttle to get it to accelerate up a decent grade or hill. This will eventually downshift the trans out of Overdrive. Plus, very low engine speeds may not necessarily deliver better fuel mileage. However, you are on the right track. We have a slightly different suggestion.
A lockup converter uses a clutch plate (arrow) with friction material that locks up against the converter housing while the plate is splined to the transmission input shaft. This eliminates torque-converter slippage and improves mileage.
Go ahead and swap in the TH700-R4 overdrive trans, but also upgrade the rear gears from those salt-flats gears to a 3.42 gearset. This will drastically improve your off-the-line acceleration and still deliver a decent 2,000-rpm engine speed at 65 mph, assuming you'll employ a lockup converter. A nonlockup converter tends to slip and will add roughly another 200 rpm to the cruise engine speed.
Combining a 3.42 gear with the TH700-R4's 3.06 First-gear ratio will give you an overall excellent launch ratio (overall ratio is trans ratio times rear-gear ratio, or 3.06 x 3.42 = 10.4:1). Your current overall First-gear ratio, assuming a TH350 trans, is 2.52 x 2.77 = 6.98:1. Of course, with a much deeper overall First-gear ratio, you will be able to fry those puny 205/65R15 tires, but that's cool. Then you could send us some burnout photos!
Heads To Go
Leroy Brigman, Abilene, TX: I just installed a 350 into an '83 El Camino that has a 0.040-over block with four-valve relief pistons, and I believe the block has been decked 0.010 inch. It has an Edelbrock RPM Air Gap intake and a Q-jet with a 1-inch adapter for the carb, a Melling high-volume oil pump, and a Melling cam with 277/288 advertised duration and 204/214 degrees of duration at 0.050 inch of tappet lift with 0.442/ 0.445 lift. The car has a Turbo 350 now, but I have a TH700-R4 in the works. The car also has a stock rearend with 3.08 gears and 295/50/15 tires on the back.
It had 882 heads on it, but it blew a head gasket the first time I drove it. I have some 487 heads that need rebuilding. Would these be worth the investment? I want a pair of heads that will run good with what I have.
My good friend has some old-style, double-hump 64cc fuelie heads. Would this bump my compression up too much for the street? Or what is a good head for what I have that will run well? This car is just gonna be driven on the weekends and to the eighth-mile strip here in Abilene, Texas. Any suggestions would be helpful in which way I go from here.
Jeff Smith: The 487 head is a relatively decent casting used in several 350 engines in 1970-71 with one of the better applications being a 330hp Camaro-and-Corvette 350 with a 76cc chamber and 2.02/1.60-inch valves. These LT-1 heads also used screw-in studs and guideplates. We'll assume you have the slightly less desirable 270hp version equipped with the same 76cc chamber but with 1.94/1.50-inch valves. We've never flow-tested a set of these heads, but they appear to be a good match for the short-block combination you described.
We'll also take a stab at the compression ratio. With the flat-top pistons and assuming a 0.015-inch deck, a 0.041-inch-thick composition head gasket, and a 76cc chamber with the 487 heads, the compression should be around 8.7:1. If all you did was change to those 64cc chamber heads, the compression would jump up to 9.90:1. This is a bit high, although not excessive, considering you have a fairly large cam that will bleed off a little cylinder pressure and perhaps keep you out of detonation. The added compression would be worth some power (roughly 3 to 4 percent) but probably isn't worth the risk.