Yes magnum manifolds on LA heads. I forgot about the water jacket intrusion. I was thinking is also easier to set the gaskets on the studs when mounting. But then again I've recently heard that early engines particularly 273 never used exhaust gaskets. Thanks I may just mount them with bolts like you mentioned with some thread sealant on the outer holes.
Keep in mind, if you start using studs that long, you will NEVER be able to remove the exhaust manifolds once the engine is installed; there's just not enough room in an A body engine bay.
And no, exhaust gaskets weren't used on LA exhaust manifolds, other than the tin heat shields/plug wire shields on some motors. For that matter, Magnums didn't use them either.
If the mating surfaces are less than ideal, you can use them (gaskets) to seal things up. Otherwise I'd just true up the surfaces of the manifolds (a bed sander works good enough for that job), bolt 'em on and go. Throw some hi temp copper gasket seal on there first if it makes you feel better.