Is the 390 motor original to the car? In Dad's garage they did a valve job on a 390 that belonged to a college student. It was marked 390, but when they got the heads off, it had the 4 1/4" bores of a 427
You could get the mach I with a 390 s code ~ 320hp.
Most I've seen had a 351w and you could get it with 2 different ones. An H code 250hp and a M code 320 hp. I'd think it would handle pretty well on the road with the lighter 351w.
But for the pucker factor you could get two different versions of the 428 cobra jet. I don't remember the codes for those but one of them had the 427 heads, and that could very well be what you were looking at in your dad's garage?
I think they were rated for like 325hp but anyone that ever hit the "go switch" on a 428 knows for damn sure they were north of 400hp.
