I wouldn't be welding on a used gas tank! (cleaned or not) :doah: Plus it's alot of work for nothin.
OK, you want that engine/carb,,,, so why not buy an engine mount plate $7 , (Or make one) and weld it to the foot bar up front and original mount in rear, then just ream out the rear holes. If your engine goes bad you'll need something universal anyway.