There are a wide array of buttons. I prefer a push-in roller. With a "push in" style button you can use pieces of feeler gauge behind the button to tighten up the end play. You can also use different timing cover gaskets if you need more play (Superformance gaskets are a fair bit thicker than stock)

There is no need for a "bolt on" button - once the cover is on the button can't go anywhere. A bolt-on also won't allow for any shimming behind it.

Teflon, Acetal, Nylon or UHMW works fine and you can make your own out of UHMW - just buy a piece at a local plastics retailer. Avoid aluminum like the plague - they will gall and spew chips into your motor if they contact the timing cover.

If using the stock cover, weld on a 1/8" plate as some have done and drill, tap and install a 1/8 pipe plug offset from center that you and fit a dial indicator through to measure endplay.

You can run very tight end play BUT you don't want zero or less or you run the risk of galling the timing gear/block face.

While on the topic ...

Do NOT use a collar on the distributor shaft to limit motion. On a BB Mopar the cam's rotation keeps the oil pump drive gear seated. It CANNOT come up and alter timing. Any upward movement of the distributor shaft itself also will not alter timing.

If running a bronze oil pump drive gear, get a thin steel shim (a .030 torrington washer) and run that between the drive gear and the bronze bushing. This will prevent wear of either.