I used the pump out of a 98 Camaro tank in my 67 GMC. I just needed a quick, cheap way to get the truck running so I put the Camaro pump in the stock, in cab GMC tank.
I filled the Camaro tank with water and cut the ring out
I filled the GMC tank with water and welded it in. It works fine until I get around to putting a tank under the bed.
A lot of people are really concerned about welding on a gas tank. It can't explode if water is displacing any fuel vapor. If there's a cheap option to just buy the tank you need, then that's not a bad idea, but I wouldn't be afraid to weld on a used tank.