The casting number of 3835587 fits all 6 cylinders except 216 engines from 50-57 (and probably later, as I was looking in the 1929-57 parts catalog.) That's the one you should probably be using. I know that 1951 exhaust outlets are sloped rearward like yours is. I also have a 1958 235 manifold set that has a sloped exhaust outlet. I can't come up with a reference to the casting number on your straight outlet manifold you said came from your 1946 216.
I have a '41 manifold set from a 216 that has the straight exhaust outlet and the wider manifold connecting bolts. I also have my 1951 216 manifold set that has an angled exhaust. That's not a huge issue that can be taken care of at the exhaust pipe. Both the '41 and '51 216 manifold sets have a spacing of 2-3/4" where they connect together. You won't be able to match a 235 exhaust manifold to a 216 intake manifold as the spacing on a 235 has a spacing of 2". There's also a difference between the '41 and '51 outer mounting studs to the engine. The '51 has a lug where the spreader contacts the manifold that's farther from the block. See attached pics for the difference. Also if you use a 235 manifold, you'll need a carburetor that matches the wider bolt spacing on the carb flange.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
