How do you get the studs out of there?

IMO, the manifold has to come off and go to a machine shop. My last experience was 15 years ago in Danbury, CT. Cost $20 then.

Also, IMO, P'Blaster, etc. don't do much to loosen the nuts, especially upside down, UNTIL you can move them a little. I have more success with heat.

I get the nut to move by working from below (one at a time), heating it with a small Oxy/MAPP gas torch till I see red, then applying force with extension & socket. (I don't think a propane torch will do it.)

Usually it will squeal and move with moderate effort. If not, I back off and repeat. Once it moves, I work it back & forth as it cools, THEN flood it with penetrant and use the back & forth motion to gradually ease it down over the rusty threads.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.