I'd love to see some pics of that 5th! As far as the 318 your best bet is to do that rear end swap first and then see how you feel about the performance. With the lean burn removed and switched for the old style electronic ignition my 5th will spin that peg-leg all day long. I think it's a 2.76 but I haven't checked, I've got a 2.94 sure grip from a friend that is waiting to go in.
Personally I would do all your mods before going into the engine. At that point if you still want more I would get the EQ heads and leave the rest stock.