I currently have a 406 (400 .030 over) with ported no.48's in my 72 Firebird- I built this engine over 16 years ago and it makes enough power to put my Bird in the 13.0x's in the quarter with a TH-350, 2500 stall, and a 3.08 posi. The engine has 1.52 roller tip rockers, TRWs, a Crane 284H cam, a Holley 750 vac.sec. on a port-matched Performer intake. I did open up the chambers to 82cc to make the heads work with pump gas, and it does run on 91 octane.
No. 48's did come on 68-69 350 HO's- provided your existing 350P is still making good compression and your leakdown rates are decent, a pair of reworked no.48's will give you a very noticeable bump in performance over those 6x-4's, even if your 350P has chamfered pistons.