C4/C6 trans.

The C4 is best behind almost any smallblock. The C6 is best behind big blocks or pick-up trucks that do a lot of towing. The C6 is MUCH beefier where it counts, but is a lot heavier, where it counts. The internals are lighter for the C4, reducing parasitic drag. You're right about the 3/4 speed differences. No OD means the engine will be spinning faster at highway speeds, technically, using more gas. The AODE can be made just a strong as the C4, which is almost bulletproof, if built properly.

You can spend the same $$$ replacing the AODE as rebuilding it. I would forget the C4/C6, unless you are building a full time drag car. The optimum solution for your situation is to buy an aftermarket rebuild AODE from Jeg's or Summit. The trans and convertor will set you back about $2000, but you will NEVER need to worry about it. A "Jasper" type rebuild will run you about $1200-$1400. Finally, a "generic" rebuild can cost as little as $700-$800. You can find the generics on Ebay and you local discount parts store. Good luck with your choice.