Just to throw an option out there for you, and this will require just as much fabrication as any other non 216/235/261 swap, you might consider the Atlas 4.2 liter 6 from the Chevrolet Trailblazer/GMC Envoy. It's physically the same dimensions as the original 216, has plenty of power, and if you can fins one from a 2wd, you might also be able to snag the matching 4L60e/4l65E transmission as well, along with all the control modules that go with it.
Alternatively, a 235 or 261 can be easily swapped in, and there are several companies that have an adapter to allow for a later model automatic (TH700R4 would be my choice) and still keep the stock mounting for the engine.
Either way, you would be looking at an open driveline conversion, which has been covered many times on this site for the assorted ways to do it. I believe that there are a couple of active threads on that very subject at this time.
