Yes, the FireHawk barrels will interchange with the same FireHawk stock, thats the way they were designed. I owned a .54 FireHawk, my first inline, killed many deer with it.

I would say no, on the interchange of the FireHawk/ThunderHawk barrels. I'd call T/C an ask, but those folks running things now don't seem to know much about the old T/C's.

As far as the thumbhole stocks you might get lucky an find one, try the auction sites or ebay.