I was wondering if a set of vortec 100cc heads would give me any gains on a 502 mag? I run a 25p mirage prop on a XR drive 1:36 ratio at about 73 gps 4800 rpms 80+ degrees and running on fumes. This doesn't seem like stock 502 numbers on a 23' stingray. only had the boat one season and not sure what's been done to it. i would like to push into the upper 70's maybe even low 80. Any input would be greatly appreciated. New to this forum and you guys seem to really know your stuff.

You can't bolt GM vortec (L-29) heads on a stock 502mag. The pop-up pistons will hit the heart shaped combustion chamber. They wouldn't help anyway...
I would suggest changing to a 1.5 ratio outdrive and 27p prop.
The 1.5 ratio will help multiply the torque.
73mph does not sound that far off though. My 24' Super doesn't run much faster with the same power.
Have you checked the Stingray web page? It has a speed estimate for their different boats with different power combos...