The 700R4 will not handle the 502 for long. Even if you get a good one. These transmissions have a relatively weak aluminum casting that is notorious for creating bearing misalignment. You can get a good one though but it is not for an engine putting out 500+ Lb-ft of torque up and down the entire rpm band.

If you can afford it (I cannot) the trick set-up is a built TH400 (inexpensive) and a Gear Vendors overdrive ($2500). This set-up will handle 1000 HP and you will have a 6 speed transmission!

Regards, Kitz