Maybe ~700+ hp is a little overboard? It would sure be alot easier, and alot more logical to settle for a ~400 hp range, N/A motor and build around that... You goto a TH400 and your talking about a BIG transmission both in physical size and weight that will be 30% less efficient than your other options. Also a TH400 is going to give you three gears, no overdrive, While the other options allow you the luxury of an overdrive 4th gear.
A TH400 is bulletproof however I have found its drawbacks to substantially outweight it's advantages... The 4L-E series transmissions are all electronically controlled (hence the E designation) and this brings up more to deal with... If you were to settle on a ~350 flywheel hp SBC you could definitely get by with a stock transmission, only modified to work with the transfer...
It is all dependant on your plans for the vehicle, your time alotted to work on said vehicle, and the size of your wallet... I don't see how you could properly do such a swap without spending big money...