318 Build Up

I've got a 1985 318 out of a 5th Avenue. It is a roller cam motor with 302 casting heads. I was planning on putting together a magnum head motor with a XE262H Comp cam (262-270 dur. 492-501 lift with 1.6 Magnum
rockers) and a Air Gap intake. Here's my questions: I would like to bump the compression up by using the small cc heads but I believe the combustion chambers are similar size. The Magnum heads do flow better but compression should be about the same.(the pistons are flat tops with no valve reliefs and are about .048 below deck) The older 318's have a open chamber head and benefit by changing to the better heads which raise compression. I also don't know if I can use my flat tappet hydraulic cam in the roller cam block.
I was planning on just freshing it up without doing much machine work.
It will be a temporary moyor while I am rebuilding the stock HP 273 in my 66 Dart. Can I use the pieces I have and make a good running small block or should I Just scrap the idea? Thanks for any feedback! Mike