i would stay away from 3076 as far too big unless your looking for a drag car.
for road and track use i think you cant beat a 2871r sized tubby (3071 is a little too laggy on circuit coming off the apex) and tends to get a lot of chatter if lightly on and off the throttle around 70-80mph, unless its externally wastegated
never driven one with gt30 fitted but i would say if your looking for 400hp then thats the one to go for as it will produce that without taking it to its limit, where as the 2871 is right on its limits to get 400
if your interested lenny ive got a brand new gt30 still boxed with actuator etc up for grabs or have an owen developments equivelant to 2871r (maybe slightly bigger) both are 400+hp capable tubbys ready to bolt up
or the 2871 from my car will be removed in a month or so and thats covered around 2,000 miles from new but is in perfect order with no play or smoke (to be fair though they are mostly track miles)