Hi Tom . . .
There are some things working in your favor with this swap. In the book Cast Iron Wonder (Floyd Clymer & Doug Bell) document that the then new 216 engine was 2" shorter then the previous generation sixes. Also, the front engine mounting arrangement was similar via a plate (Parts Master Group 0.004):
Old Car Manual Project - Master Parts Catalog page 94 .
The plates from the 216 have been used to transplant the newer 235 and 261s.
With a shorter engine the transmission mounts will need to be reworked. What gearbox/tranny do you have in mind?