Yes, you can use a flexplate with a Stellings driveline setup but only if you have a bottom mount starter. PM me if you have a top mount starter and I could build you one with the ring gear in the correct position.
GF?

Does your 502 have the stock rotating assembly ?? externally balanced

You can get an External balance flexplate. Not sure why the flywheel is bothering you though. A flywheel, with a heavy duty damper plate, should give you no issues at all.

Agreed, you can get a neutral or external balance flexplate but you cannot get an off-the-shelf flexplate for a top mount starter. His Stelling extnesion box setup has the u-joint flange bolted directly to the crankshaft bolts with an adapter. So, no coupler or damper plate. IMO, not really the best way to do it...

GF?

what is the reason for wanting to go to a flexplate?

won,t pizz me off,what you are considering is doable,you would need to add a spacer between the crank&flexplate if you have a reverse mount starter,also i would use a heavy duty sfi approved flex plate due to it,s extra strength.

My biggest concern would be with the runout on the flexplates. Every external balance flex plate I have seen typically has quite a bit of runout due to the weight being welded on. I see it when I have them on a crankshaft in my balancer. They typically wobble back and forth up to .100" Once a converter is bolted to them it is really not an issue because it gets pulled straight. Without something bolted to it it would have way too much runout for the starter. Not sure if a good heavy duty one like ATI would be any better or not.

As far as getting rid of rotating mass I dont know if that is really that important in a more constant rpm application like a boat. The extra mass might even be helpful in keeping the rpm more constant in choppy water ?