Ok, got the studs out. Propane torch with a turbo option was just enough to heat it up. I also took some other advice and used some candle wax on the stud while it was still hot.
Seemed to do the trick as the studs came right out using the double-nut method.

Also, (for custom making of new longer studs) when I tried to die cut some threads in the spare bolts I have, seems they are 3/8" not 5/16" ..at least that's what the die stamp reads -- diameter measured at .354"
So it now seems, manifold: 3/8-16, carb: 3/8-24.
After applying some no-seize compound, I'm ready for reassembly.

Thanks again for your help shake