The 700 R4 has a couple of weak points, but if you're not going to abuse it, it will give you better gas mileage. It's got overdrive, and a lockup torque converter that eliminates the 10% slip that all converters have as a factor of the physics involved in being able to multiply torque. The overdrive clutch is pretty puny, but it can be replaced with the clutch drum intended for the Corvette which has bigger clutch plates, and more of them. If you're planning to tow anything, stay out of overdrive. The OD clutch cycling in and out every time a load goes on will wear it out, and also overheat the fluid. Use the biggest auxiliary transmission cooler you can fit if you go with the 700 trans.
350's are plentiful and cheap, and there's a lot of hotrod parts available for them if you plan to get radical with the power. The best choice for a high-performance package is the Turbo 400. it's a little heavier than the 350, but MUCH stronger. If you can figure out the wiring without going to a full computer hookup, the 4L80 is a 400 design with overdrive and a lockup converter, but it would be a bear to wire up!
Jerry
