I would either buy a complete package with heads and cam and ecm recal for the package or just leave it stock and have Dustin Whipple recalibrate the ecm. Whipple seems to know a lot about getting the absolute most from the stock 500efi by adjusting the timing curve, fuel curve, and rev limit. You might pick 1-2mph with just the recalibration.
I would still of course replace the springs and seals and get a valve job. And yes, new ARP head bolts.
From what I have seen and read 600hp is about the max with the 500efi intake.

so can you change the intake and leave the stock heads and gain hp and torque

Do an entire package upgrade or leave it stock. If you don't have more flow through a better set of heads, then you don't need an upgraded intake.

got ya. makes sense. thanks for the help