[Kim57]

Denee
Removing fire wall flange: If I were you I would remove a little at a time and then weld that part then move to another section and do the same so you wont change how the cab fits to the fire wall. It will take some time but IMHO be worth the effort.

Removing cross member: It should be riveted in from the factory. You should be able to grind off the heads and then use a drift to punch them out. That way you would be able to re-install it if needed. Mine is gone but I have a drive shaft loop that ties my frame together.

Engine mounts: I don't see why you can't cut off the top part and then weld and box it all together to achieve the smooth frame that you want.

Others may have some different opinions about these items.
Kim