The 6 speeds typically handle less power than a 5 speed unless you spring for a viper spec T56. As said before 700rwhp is a a lot for a stock block to handle but anyways, you may want to look into an aftermarket tko600. I think MMR builds one up to handle a lot of power like that.
Kevin
With that much power if you are plan on racing it.A Auto would be a WAY better choice..
A friend with another 600rwhp car went from high 10's to low 10's by going from 5 speed to auto .And the car launched better(1.49 60's on drag radials--its a street car).
AS far as a manual that will hold up.Personally i say tremec period..Hell i had a old school tko rated at 425tq that help 600+rwhp and 500rwtq for over 5 years before it finally gave up.With the newer version they should hold up even better.
tko 600 will do plenty.
Hell the regular t56 is rated at 400 hp and plenty of terminator guys are pushing 600 hp with it.. imagine what a tko could respectively hold