Engine bench test

Use a flywheel with the proper balance.

Nonsense. You are not going to be winding the engine up very high or for very long. "Back in the day" (70's--80's) I ran a neutral balance 340 wheel behind a 360 for a couple of years in an old Landcruiser, and ran it pretty hard, too. Yeh. It shook. It was LONG before Al Gore invented the internet.

I don't even use a bell. Just a flywheel and the engine plate off the scattershield. But you could MAKE that if you can fabricate. Just use the automatic for a pattern. If you are afraid of breaking the starter round up a junkyard cheapie and gamble with that. So far, so good