Swapping a 65 273 with a Later 318. What's gotta Change?

The '65 273 has a small snout converter that won't have the proper support in the bigger, 318 crank register. There use to be a reducer ring available, but one might have to fabricate their own.

You can't swap converters in the original trans to a newer one because the splines are different, unless you get that one modified or get a custom one made. Otherwise, you'd have to get a later model trans & converter to match the 318. I don't know what the OEM shifter linkage compatibility is.