Good recommendations from 2000xp8. Those trans are the ones to have. I am personally a fan of the T56 and T56 magnums. The T56 would need the 26 spline input and the 30 or 31 spline output, though its internals should be fine at that power level. The TKO 600 should also hold up. The major advantage of the T56 is that if you go with the .74, .5 ratios for 5th and 6th, you can put a really, really short rear gear in and still cruise at a nice highway RPM.
If you got one out of an '03 Cobra, it would have a 10 spline input, 27 spline output, and a .63 6th gear, which is not a significant difference from the overdrive in the T5 or TKO (.675).
Just curious, how are you planning to make 700rwhp? Because, I've got the sneaking sensation that you think you'll be doing it with a turbo on the stock block. If that's the case, save a lot of money an time and either aim for 500 rwhp or sell your car and find one that's already built right for that sort of thing.