Have factory original 454 mags 1988.

Redoing the heads and a valve job. Is there a cam that would give me a little bump in power. Nothing to extreme but it would be nice to get a little more power out of them?

Only one I'd personally recommend from my own personal experience if your not making any other changes would be the 454/420 hp and roller rockers.

You really can't do any significant cam upgrade unless you have performance exhaust with longer risers

Had the same engine and griff is correct. Crane 731 roller cam, lightning headers, port heads = 450hp.

89 454 Mags and I went with roller rockers, Comp Cams Extreme Marine hydraulic cam, heads refreshed, Edelbrock RPM air gap intakes and Holley 750 carbs. I was told that the Mercury 496 exhaust would support up to about 500hp and is the route I went finding new take-offs pretty cheap. That said I saw about a notable increase in top speed.

The 420 cam and roller rocker upgrade wasn't significant however was btwn 3 to 4 mph diff with same prop. Enough to make it worthy not a big gain by any means. That was with stock manifolds and risers.