They would work on 3.1's correct? Would 45 PSI work on a 3.1 with full exhaust? or would it need to be more,like 47-48?

The tpi regulater will work with the 3.1 dunno about the fuel pressure tho

My friend has an 89 TPI 305 and I could just use his regulator and gauge.Im not too sure what to bump it up to though,if to do it at all

It's the same part for V6 and TPI.
Here is my fuel page, it has the FPR that I used on my V6 with the summit part # too.
The stock fuel pressure is 43.5psi, turn it up if needed.

To 45 PSI?

Any other modifications to pumps/injector/rails needed?

Turn it up as high as you can until youare blowing black smoke out the tailpipe, then turn it down a notch below that until it stops.

All you need is to put it on there and turn it up untill you run lean then you will need new injectors then a pump as some point. Maybe not with with a V6.