For your Craigslist consideration today here's a four-barrel cast iron intake manifold, GM casting number 3746829, for $160 cash FOB Pomeroy WA.
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Original equipment on the 283 Corvette 1958-1961.
This one in particular was on a Chris-Craft marine engine until I pulled it off in 1998. It's been on my parts shelf for the last 25 years.
No breaks, no cracks, still sporting remnants of the previous intake gaskets.
Carb bolt pattern is:
5-5/8" along the crankshaft axis, center to center
4-1/4" across the crankshaft axis, center to center
Bores look to be 1-5/16".
I believe it's a WCFB bolt pattern.
I do wander over to the L/C area from time to time and could bring it along if that helps.
Thanks for looking!