I just measured the studs in two manifolds stashed away in my storage building. The 216 manifold has 3/8 X 2 1/2" studs, and the 235 studs are 3/8" by 3" long. The thread going into the manifold will be UNC- - -3/8-16. The end with the nut on it can be either 16 or 24 thread- - - -just use the corresponding nut. A lot of standard-type studs available at a FLAPS will have a STD thread on one end, and a fine thread on the other.

Chevy small block V8 exhaust pipe studs used with a heat riser valve will be almost the exact length you need for the stovebolt manifolds, and a set of brass nuts for them and some anti-sieze compound will keep the threads from rusting together again. Just take your time getting them out!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!