The price of street gas being what it is, even the most hard-core hot rodder must grasp the consequences. Certainly, they can curtail driving to reduce fuel cost, which often precludes traditional destinations and may even cut into appearances at the local drive-in. There is a sensible way to combat this issue, even if it only means a few miles per gallon. Overdriven top gears have been around for many years, mostly at the behest of the OE manufacturers-see, they actually did learn something from the fuel-blank debacles of '73-79. Overdrive automatics became standard equipment a quarter-century ago. It's about time hot rodders got hip to this worthy upgrade. We can't believe anyone would still want to "cruise" with straight 4.56:1 gears.
The abuse the engine suffers at 3,500 droning rpm is a lot more worrisome than one gliding along the high-speed at a sedate two grand. Think how much quieter things will become. Think how much you'll reduce the size of your environmental footprint. The cruel fact is that as much as hot rodders have historically scoffed at the rules and the socially accepted norms of the period, we have to start playing the establishment game, at least to some extent. Here's one way to do it.
Though the notion seems simple enough-replace a transmission with a similar, albeit electronically controlled, one-all is not what it appears to be. Will the bellhousing attachment holes correspond to the ones on the back of the block? Will the driveshaft bolt back in? Is the hardware compatible? Before you begin, seriously consider using new fasteners available from any GM dealer or aftermarket source.
Quick Notes
What Is It
How to tell the difference between factory overdrive transmissions.
Bottom Line
The information is here; the rest is up to you.
Cost (Approx)
Contingent on your choice.
 GM used two types of torque...  GM used two types of torque converter-to-flexplate fasteners. The black bolt (left) is metric (10 mm x 14.3 inches x 1 1/2 inches) with corresponding nut. The gold bolt (right) is standard GM (3/8 x 1/2 x 16 coarse). |  The thread size for all cooling...  The thread size for all cooling lines in all GM transmissions is identical (5/16 x 1/4 pipe thread). During the '73-74 model years, GM enlarged the head of the fitting to discourage corners from being rounded off and rendering them useless... |  ...The large head version...  ...The large head version (left) was used on Turbo 350, Turbo 400, 700R4, and 4L60 through the '96. Thereafter, the fittings were omitted in favor of a snap-in type connector. |
 The driveshaft yoke for a...  The driveshaft yoke for a 700R4, 2004R, 4L60, 4L60E, and so on (on the left) has 27 splines and is obviously narrower than the 4L80E yoke. |  The Turbo 400/4L80E yoke has...  The Turbo 400/4L80E yoke has 32 splines and has considerable heft and girth compared to the lesser yoke. |  If you're going to get really...  If you're going to get really rough with the car, then perhaps a sturdier polyurethane mount (in this case Energy Suspension) would be in order. The tailshaft bolt pattern and crossmember attachment are identical to OE pieces. |