I think just a hell of a lot of back n forth, up n down, heat, penetrant, prayers, voodoo dolls, whatever you’ve got.
Going back to the intake manifold bolts and their holes I would not hesitate to check the threads and run a tap down them, bottoming tap if possible.
You’re in territory we’ve left long ago. But our memories aren’t so faded. Those intake bolts and their holes get a lot of corrosion no matter where you live and they are notoriously difficult. Given you’ve got yours out comfortably maybe you’ll get lucky and they’ll seat with enough torque again. But I would not place good odds on that happening. For sure you should use some Loctite.
What I’ve done and others is use an insert, not a heli-coil. The Brand I use is TimeSerts and they’re a little more but you’ll be able to remove your bolts with ease and send them back in again. Your choice and I don’t blame you if you just use Loctite and call it a day. Just be aware those are old and we’ve been there.
I would not count on any old sealant that’s been cured for ages resetting itself at this point. I thought those were just pressed in.