undy,
By any chance is your intake a Dove? Mine is. Very early first batch in the '90's.
Looked like the surface of the moon when I got it. P'test revealed no water leaks however some port runners were paper thin in spots. My solution as follows:
Bead blasting to get a clean surface and 100psi air clean-out. Then used JB Weld like a 'spot putty' to fill divots and smaller pock marks. File and sand to suit.
Once cleaned up, sent it for ceramic coating silver with instructions to try to keep the oven below 400 deg and just let it gloss for a longer period. Worked perfectly-did not lift the JB. I don't think the ceramic is dense enough to fill your crack or fissure, but like Barry, have no practical experience doing that. I do know that JB, (if you can 'vee-out' your opening and have access to apply it) will withstand at least 25 psi as your system is only 16 to 18 psi.
I have no experience with the internal sealing systems the other guys recommend but JB has been a part of my intake since 1991.
BTW, have JB in small areas of block where 250 deg temps and
oil drenching are prevalent. No problem either.