I have the engine out of my 76 and have aquired a set of SSI heat exchangers. The old heat exchangers are still functional just not as nice as the new ones.
My fear is that I risk breaking some exhaust studs when I try to remove the old ones even if I soak them with penetrant and judiciously use heat. (I'm thinking a 20-30 percentchance off snaping one.) Am I better off just leaving good enough alone and wait until I plan to pull the heads before installing the new HE's
Any advice is greatly appreciated.

Just do it! There's your answer.
Should we start a poll?
If you screw it up now, you can have the heads fixed this time.

+1 just do it....at least you have the engine out and if you do break something, it'll be easier to
do it now than after the engine is back in....

Wow! That is a tough question. I have always believed if it is not broken, don't fix it! That is just me. You could "gently" try loosing the nuts. If they don't want to come off, go back to my rule! Pelican Parts sells replacement nuts. Good Luck!

Try removing the exhaust nuts and if you run into any issues, leave well enough alone and keep using the old system.