The old CAT Shop Notes have a nice tech tip to use when reinstalling the heater core .
The bulging of the end caps as most people know happens because the unit was built for the alpine cooling system that runs at considerably lower pressure (6 pounds or so?).
To prevent reoccurance of bulging and resultant leaks after a rebuild, you can strap a couple of square pieces of steel, like 1/8 inch sheet, the size of each end cap to each end with a couple of worm clamps. The bands encircle the entire core horizontally. The corners need to be bent into 90 drgree turns so a bit of trial and error to get that part right is involved. I used stainless steel ones for metal ventilation ducts from the hardware store. As I recall I had to put the worm part at the back in order to slip the entire unit into place. It just slipped into the hole. So far no more leaks.
Gene