You will love the 347, really pulls like a mother in the power band. Just curious why not wait until you finish building the short block (heads, water pump, intake etc...) before puttng in the car? A lot easier than bending over the fenders. My guess is you just couldn't wait, which I can understand ;-)
to be honest you are way better off just taking the hood off now, pulling the motor out and installing the complete engine and trans as one assembly. It will make your life much much easier. I installed my completed 347 with the deep pan and Tremec 3550 at the same time so there is plenty of room.
I agree with installing the engine assembled. I installed my 302 with everything including the clutch except the air cleaner on it. Of course the hood was off. There was enough room with the 1 peice export brace installed.