I've used:
Ray’s Radiator Clinic, Inc.
6550 Cobb Drive
Sterling Heights, MI 48312
586-759-5010
They hot-blast-acid-clean the inside of the tank; if they can't get to all the rust inside they cut holes in the bottom of the tank to gain access then weld it back up; in the process they weld up any remaining pin holes; then they coat/seal the inside of the tank and media-blast the outside - $275
Failing that - I've got a complete new Caswell tank sealing kit I'll sell you - $50
David Hicks