There are differences. The Heat riser bolt pattern is different. The intake port diameter for the 1950 and later 235 is 1/8" larger than for the 216s and the pre-1950 235s. The master Parts manual gives the part numbers and some physical data on the manifolds and related parts. The 216 manifolds will fit the 235 head, but the stock intake port alignment rings cannot be used. The 235s also have a larger bolt pattern for the carburetor. I've never seen the insulator-spacer melt, but I've seen them erode and cause vacuum leaks.

Hoyt