Ritchie, Beggar's Performance rules!! I see that You should make a mock up, trial fit before assembling the truck. Before the work really begins, how is Your working space? and tools, do You have how to move the engine/trans combo easily? Any good way to drop the freshly painted cab over the chassis without ruining it? If You have it all, and some space to work freely around the truck, I'd work in this way: First, place some wood blocks under the engine in the chassis, then try to slide the painted cab over it all and see what goes on. If fitting is really ok, make some templates to place the tubular supports of engine and trans the right way in the chassis.
Probably You'll end up with a lot of space, as long as the mouse is much shorter than the stovebolt. I see that if the radiator is in good shape it'll handle the 305, that for sure is lighter than the 216. But do not forget two issues: You'll need a good fan shrouding to make it work properly, and also the trans cooler, 'cause probably the old radiator does not have one.
