360 HP estimate???

I'd guess fuel-air delivery, too. Numbers with that tire diameter and top end speed works out to 5500-5600 RPM with a 4.56 rear gear. So something is choking it off; 600 cfm ought to do better than that.

Why not go with a bigger cam to take advantage of the higher CR pistons and then you can go to better heads? Can you confirm that these are KB190 pistons? (Which show as 10.5 at 72 cc's...)

You can use better heads if you work it out. With those pistons and stock deck height, and a .075" thick Cometic head gasket, Edelbrock 60779 heads should give a good quench gap and a DCR of around 8.1 with a 282 advertised cam. You can tune around that for sure. Just check the piston-to-deck height if they are KB190's, to make sure the piston to head clearance is adequate. If the block has been decked, then look at the Edelbrock 60179's with the .060" recess and vary the head gasket thickness.