Try asking this here
http://forums.performanceyears.com and you should get plenty of help. Any Pontiac engine will fit so it is not hard. In 1964 the starter moved from the bell housing to the back of the engine so if not caring what engine you use go with any 64 or later engine/trans combo. Drive shaft will have to be shortened or changed depending on the trans. I would go with a T-350 or T-400 as the old slim jim 4 speed autos are not the most reliable and won't take much hot rodding. When you cruise you willl hot rod it and don't say you won't.
All heads interchange with blocks beginning in 66 and T400/350 will bolt to all of them. Just has to be BOP trans.
Also check your local craigs list for one as they are usually cheaper there. Finding a 2 dr ht or post will not be cheap inless you 'find one in a barn'. pm me if you want.