If you have 600 hp I'm guessing that you have a fairly radical cam, so at least a 3000 if not a 3500 stall. Are you running regular size tires or the big pro street type tires? It will make a big difference if you are running a 26-28 inch tall tire versus the 31 ot 33 inch tall tires like a lot of us run. If using a 26 inch tall tire with say 3.08 gears, to get the same rpm with the 31-33 inch tall tires would take a 3.55 or 3.73. Everybody hollers a 700R4, but if you do have a BB with 600 hp and the torque that comes along with it, stay away from it, because every trans builder I have dealt with tell me 400 hp is the max they will stand even beefed up in the long run, fine for stock BB or the average SB, but not with what your running, with any power at all I don't know why you need an overdrive in a car this light, it will pull a high gear without any problems, and just is something that adds to the complication factor, best to keep it simple. For reference my bucket has a high rpm tuned 331 sbc and can turn 7500-8000 and not hurt it, and I am running a TH350 with reverse manual valve body, and a 3500-3800 stall convertor with 4.30 gears, and 31 X 18.50 M/T street legal slicks, have to look hard to see miniscule tread, and it is great for performance, and if you keep your foot out of it the guy I got it from said on a 400 mile trip it got 18 mpg, I don't care about fuel milage, I never drive any extremely long trips anyway, the fun factor when you hit the loud pedal is what I like!
P.S. I have never known anyone that used a 700R4 with any amount of power that didn't break it on a regular basis, and the linkage that controls the valve in the trans is so sensitive that it is very easy to wipe the trans out if it is not adjusted perfectly, will never catch me running one, and my trans guy I use refuses to build them because of the breakage record, and he builds for many of the top name Pro Mod and Top Sportsman racers here on the east coast and from all over the country.
