Couple of things I ran into when putting an 87 318 in my 65 Cuda. The 273 exhaust manifolds hit the bump cast below the exhaust port for air injection, had to grind it off. The manifolds also do not fit easily. The exhaust port in the heads is almost bigger than sealing surface on the manifolds, careful positioning is needed there. Obviously you have the issue with the torque converter snout being smaller than the centering ring in the crank.
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.
