Gonna need to know a whole lot more about the turbo to really tell ya.
What series compressor wheel? and what trim? What size?
What a/r on the turbine?
What size motor are you running? Heads/compression, etc.
Nitrous will increase the HP as well as cool the charge.
Uhh, what made you buy a 76 just for the 4" inlet? There are plenty of cone filters with 4.5+" mounts, then again you probably wanted to know that a while ago huh