You can get a 700/R4 that will stand up under 450 HP for a little over $1000, and up to 630 hp for about $1300. A matching converter goes for $270 - $570.

In a 700/R4, you NEED, I repeate REALLY NEED to use a locking converter. If you drive it on the street without a 4th gear lockup, you will burn it up in short order. All the trans suppliers have wiring kits.

Homework assignment:

Read a bunch of stuff at http://www.bowtieoverdrives.com