well, you're going to need to do some port clean up on stock magnums to hit 425+ HP, nothing major, Dulcich had an article in an '05 mopar muscle with some minor bowl, valve job, and port pinch work that added ~25CFM to flow on some maggies, which would yield them to support 450-460hp.

one thing to remember with the reman heads--you'll still need to buy new springs/retainers for any type of performance cam, so that'll add more cost, whereas CME engines sells new enginequest heads w/ chevy +.1" long valves, 5 angle valve job, and springs that'll handle a hydraulic roller up to ~.550" lift for under $900/pr. if you're into some reman heads for $350, and have to spend another $200-300 for springs & retainers (hard to find a low cost spring/retainer solution for mag heads, unless you can find a few 2.2L/2.5L mopars in junkyards to yoink the retainers off of), it might be better just to go new/chev 11/32" stem valves.

what kind of R's do you want to turn? maggies have a small cross section, and they'll make their best power on a 408 on the idle-5500 RPM range, so I'd cam accordingly (probably a roller cam similar to the crate motor cam, but with a little wider LSA, personally). you might want to save the $$ and just use some 0 deck KB107's and the stock stroke.