3.6 in A-body
the real issue with putting a 3.6l in a more narrow 66 is the exhaust
I struggled with it in my wider 1972. I would be concerned its even possible on a early A-body.
The center exhaust ports dump exactly in the narrowest part of the chassis. The Inner fender blocks it on the right.
The steering coupler is even closer on the Left. Forget about power steering unless its a rack & pinion.