Update- I just resealed the manifold with 3M weatherstripping and gasket adhesive and so far so good. What I did is remove the gasket entirely and just smear a thin coat around the ports. I did not start the engine yet, but after letting it dry for 1 hour I smoke tested the manifold (at very low pressure so I did not disrupt the sealant as it dries) and found no leaks, so for now it is sealed. Whether it remains sealed after driving it I do not know yet, but I will report back. I am gonna let it dry until at least tomorrow night before starting it to give it the best possible chance of working.
Also, that stuff seems to have at least some fuel resistance. I put a small amount on a piece of plastic and poured a little gas on it after letting it dry for 2 days and it seemed to be unaffected, at least in the short term. It did not seem to dissolve or get gummy. Time will tell how well that stuff holds up, but I will be sure to keep you posted.