Okay, before I jump the gun, let's specify what we're looking for here.
What year Monte? There were more years that came from the factory already RWD than the 95+ FWD Montes. Are you trying to convert a FWD to RWD and add a 5 speed or just swap 5 speed transmission into RWD Monte ?
A 700r4 is probably the most readily available automatic overdrive in existence. I would not call them rare. Also they never made a "manual version" nor was it ever a 5 speed. So that begs the question, do you want a 4 speed or a 5 speed? Automatic or Manual?
"4.3L possibly as large as 454" You're going to have to be specific what engine, brand, displacement, cylinder arrangement and modifications you plan on so we can narrow down what is the best choice for you. Also you need to take tire size and final drive ratio into consideration.