Is it acceptable to set cam end play with timing chain off, but cam gear on cam? I ask because my cam has none to very very little movement with chain on.

Also, I thought about welding something on the outside of timing cover to be sandwiched by the water pump housing, to stiffen the cover? Sound ok?

Check it like it will be used

Cam buttons are available through Mancini as well as others. Don't know if that's what you are looking for or not.

Setting it with no chain on is fine.
Are you running a roller cam? A roller cam needs something to limit its movement. A solid or hydraulic cam does not.

Finding an off the shelf button that will fit and give you the right clearance is virtually impossible. Avoid the nice, expensive roller bearing style because these can't easily be clearanced to fit. Avoid aluminum buttons because they will gall. The nylon ones are the best choice since they can easily be ground or filed to fit and the finished surface doesn't matter. If a button is too short (too much end play), cut a piece of the correct thickness feeler gauge and put it behind the button.