300 crank from a 5.2 is supper easy. I think an air gap nice carb, headers, nice exhaust system and a small upgraded cam will do the trick no problem all for under 1g
I would run hedders, but in my engine bay (early-a) space is limited. I have 273 manifolds on my 318 with 360 heads in the car now and they suck flow wise.
No head work needed except lift clearance. The retainer to seal should be checked and clearances made if need be. The 1.6 rocker ratio ... is it figured in with the lift mentioned? I see it sez roller cam. So, I'd like to assume it is a cam for the Magnum engine/head.
Porting wise. Leave the heads alone. Use the parts you listed and you will have what you want.
As far as exhaust goes, off the exhaust manifolds you'll be running, just do it simple. Dual exhaust with a X or H pipe @ no larger than 2-1/2. Mandrel bent if possible.