how much horsepower?

P.S. with 20-25 psi you'd need a heavy duty r block based setup. You'd need the extra head bolts FOR SURE and I would recommend EFI and the enlistment of a good tuner. a factory block wouldnt live through that.

20-25 psi would be 4 digit HP. LOL I would be scared!

12-15psi and you would smack the living crap out of anything on the street. lol
The r block can do about 1500 hp.
I wouldnt even try more than 18psi and that's bold IMO.
I dont say that because it cant be done. I say that because my skills arent at that level to drive that, or to tune it.