The bolts which go thru the heat exchanger are made out of two different materials and are subject to corrosion.
I had a similar problem last year with a Volvo engine: the heat exchanger was broken and needed to be replaced
but the damn X&%#$@ bolts wouldn't budge. Have you tried using a penetrating spray? That _might_ help. I
recommend Aero Kroil rather than PB Blaster. More expensive but seems to be more powerful.

Jerry

PS: you don't want to hear how the heat exchanger on my boat was finally removed and replaced. Good luck!