First off, not that it really matters, but if the engine is a 350 & its from a 78 Mailbu. thanb it wasnt the Malibus original engine. unless Im mistaken the only v8s available for 78 in Malibu were 267 & 305.
now, as far as your current engine being gauranteed 350 HP, based on what specs you gave me,n ot a chance, so punched it up in DD2000 just to see what it said.Hell maybe I was wrong LOL. heres what I came up with.
peak HP 294@4500 RPMS
peak TQ 370@3000 RPMS
after switching cam, intake carb, & heads( by the way, 76cc to 64cc gets you 10.7:1).
peak HP 346@5500 RPMs
peak TQ 382 @4000 RPMs
Those numbers are assuming you have headers & a decent exhaust system, you didnt say.
You said it has Performer intake now, & plan to change to an Air Gap, I assume you mean th RPM AIr Gap, its confusing, but you can by the AirGap in Performer & Performer RPM.
If that was the combo I was going to use, I think Id just keep the regular performer on it, & either keep the 600 cfm carb & add a 1" open spacer, or buy a Demon 625 for it. John