About 15 years ago I made my own intake manifolds for a later 250 inline 6. I made 3 individual intake manifolds, for stock 1bbls those engines ran. Carbs were available at Kragen at the time for about $25. Each manifold consisted of a plate on the head that was close to the shape of the original manifold, with a pipe angled at 45 degrees, out and up, then a flange on top to match the carb. I made one to fit each intake port, 3 total since they are siamesed. The engine had a big cam from Clifford, a 194 head for higher compression, about 11:1, headers and dual exhust with glasspacks, with a Muncie 4 speed behind it.
I never could get it to idle well, I know now that my idle feed restrictors and idle air bleeds were too big. They were trying to feed 6 cylinders and I couldn't shut them down enough for 2 cylinders. The other problem I ran into was the carbs would ice up on a cold day. I had not provided heat to the manifold in any way. It does need SOME heat for atomization. If I were to do it again now I would run a water pipe across all the carb flanges from the water pump or something.
Oh, almost forgot, I sized the pipe the same diameter as close as I could to the port size. Not very scientific, but it worked.
Hope this helps,
Devin
