A 270 at 6lbs of boost (~1.5bar absolute) at 5500rpm and a 90%VE is ~580CFM (40lb/min air) and roughly 380HP with ideal tuning.
That makes a few assumptions, but it is hard to avoid that without a lot of expensive equipment.
So when you are looking at turbo maps, you want to find one where 40lbs/min air and a pressure ratio of 1.5 fall in the high efficiency area. Something like a Garret GT40R would work, but it is going to spool fairly slowly because 6psi is near its bottom range.
