I have an 87 318 I put into my 65 Cuda, the overlap between the 87 exhaust port and the 65 273 manifolds is very small and if not carefully attended to you will have a leak.

Test fit the manifold to the heads and see how well they fit up, outside the car if possible. I ended up using the 273 steel shim exhaust gaskets as I felt they would be less likely to blow out.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.