Have you tried any of the flush chemicals, I had a chevy II that did the same, had to run 3 time of flush before it broke loose. the next week i had to replace the water pump again (chemical eat the seal)........LOL should have put the old pump back on first. If not the top and bottom tank should only brazed, and crimped. If you remove them you could run a rod thru the tubes. If you don't want to rebraze the tanks you could epoxy them back.
Bradc