[SFfiredog]

Even if you could fit medium riser valve sizes in a 390 bore, the valves would be so shrouded by the cylinder wall that flow would be decreased instead of increased.
Recipe for a 500 hp 390:
.030 390 block
Scat 4.125 stroker crank
431 ci
10.5 to 1 compression
mechanical flat tappet cam with specs around 248 degree duration a .050" with about .590 lift
Erson rocker assy
Edelbrock Performer RPM heads w/ mild port work
" " " Intake
MSD distributor and 6AL ignition
750 to 850 double pumper

Should make an easy 500 hp and still be easily streetable.
--Mike