How 'bout a stepped stud that's got bigger threads on one end and smaller on the other? Dorman #675-097. It has 3/8"-16 threads on the head side, and the stock 5/16"-24 threads on the manifold side. Overall length is a little shorter than the 675-092 standard replacement stud, but that's of no consequence. That's what I wound up successfully using at the end of a long comedy of ignorant errors and dumb mistakes* a long time ago on my '65. All you have to do is overdrill the hole straight and true. Head on engine: Not a problem. Manifold on head: Not a chance.

Use Loctite (not RTV or teflon tape) on the head-side threads, which means you need to clean the coolant and tapping oil out of the overdrilled head hole using something (alcohol or brake cleaner, e.g.) that won't leave a residue. The head threads are 7/16" deep on the 675-097, while the original-type 675-092 has 9/16" of head threads; if you're careful and mark your tap accurately with a loop of masking tape, you can make (leave) a ledge at the "bottom" of the overdrilled head hole to bolster stud seating and sealing.

*-involving a hammer drill, "EZ-out", jigsaw, tap extractor, JB-Weld, Plexiglas heater, and assorted other inappropriate tools and supplies