"I guess I'll try mapp gas heat but I worry about too much will damage something else?"

Aim at the bolt and away from the pistons which have rubber dust boots. I applied heat, counted to 30, didn't work. Next try to 40, didn't work. Next try to 50, worked.

"Don't you have to remove the caliper to install new pads?"

No! Squeeze the brake pads against the caliper (away from the rotor) using something like vise grips. Remove the small hairpins from the pins (threadless bolts) that pass through the "ears" of the brake pads. From the front, knock the pins out with something pointy and then, from behind, grab them by their heads and pull them out. Expect the retaining springs to go flying. Slide out pads.
http://www.fcpgroton.com/product-exec/product_id/9452/nm/1985_1993_Volvo_240_Front_Brake_Hardware_Kit_Vented_Rotors_/category_id/48

"someone mentioned replacing the 15mm bolts with 17mm."

I think I used 19mm bolts. Match the thread, diameter and length and get the hardest.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.