going to pull the heads off my 500efi this winter most likely, just trying to see the best place for parts and if i should run with the oem mercruiser, or is there another brand guys are running with? (talking about valves, springs, head gaskets, seals, exc.)

Order your parts from Eddie at Young Performance. He will know exactly what you need.

Plenty of better parts than the stock Merc stuff.

Definately, call Eddie he did my engines. He will sell you better parts, for less than it would cost you for stock Mercury parts

Thanks Griff and John.

Give me a call Prostock. I would be glad to fix you up with some parts. For your application, I like to use Isky springs, Comp 10*retainers and locks, and flouro-elastomer seals. The stock retainers and locks are 7*. The 10* stuff costs a few more dollars but the locks are much thicker and stronger. It's just cheap insurance. Not to mention, I do not keep or sell 7* retainers. I just don't believe in them.
The parts that I mentioned will run approx. $300 shipped to your door. Like i said, give me a shout if I can help. Thanks.
Eddie