500 hp needs about 250 lbs/hour of fuel which is roughly 40 gallons per hour. So your existing pump should be fine if it can actually flow 65 gph at 6 psi. A free flow test doesn't tell you what it flows at 6 psi.

If you're going to switch tanks then buy one with an internal EFI pump. Those are nice and quiet and they have plenty of flow. You'll need to add a regulator and a return line but you'll have a top notch fuel system.

A 240 lph EFI pump should be all you need and they are fairly inexpensive.