I think there would be the most complcation is getting the engine mount
right.   Yould need to use a case from a 68-71 bus with the taps for the
mount, or a case that had an adaptor on it.  

The other thing is to adapt the engine tin, as the 1600 tin will not fill up
the gaps, resulting in over heating.  pop rivet on some tin or something to
keep the engine compartment intact.

I remember some one expressing the opinion that the later trans is
incompatible, as the trans shaft is longer, but i find this hard to believe
as 1) the 1600 was an option in 72-79 buses in some countries,  2) I took a
1600 out of a 72-79 to snag the tin,  3)  bug drag racers use 77-79 trans in
the race bugs as they are sturdier

With the comparable prices of engines,I'm surprised more people dont do it.
 (unless you live in California, where they'll hang you from your toes and
flagellate you with plastic snow plows if you dont have correct smog
equipment for your year)