If you are only gonna race it or only drive in short hops I would stick with a TH400,, if you plan on driving it a fair amount including distances I would not use anything less then an O/D, and in my opinion if streetability is important your hp of 550 plus may be to much with a carberated engine to have street manners. ( Idle and vac issues should be quite fun) and the stall converter you will need to keep it happy will make all standing starts feel like a standard with a slipping clutch.
Alsoi with that kind of power you also should look at changing the rear end totally including half shafts and joints . I am pretty sure an aluminum differential will go off like a bomb at some point during hard acceleration
And the 700R4 will fit like a glove in the tunnel , if you go that way get a cross member from bto and google shifter detents for the company who makes your 3spd stock shifter a 4spd shifter