318 power?

Well, we picked up the engine from a guy on Craigslist along with a 727 TorqueFlite Tranny. The engine is in great shape (zero decked, cross hatch markings still visible.. etc..) . All we know about the cam is that its on the larger side (its all dark and has yellow on the end connecting to the distributor). The engine was run before, so at least we do know the cam works. Also, we just took in the tranny and had it fully rebuilt and a TransGo kit put in. We are using a torque, not stall converter. It has the stock 2.76 gears in the diff (peg leg). Now the question is: Should we get a Posi Sure-grip diff with 3.23/3.55 gears or just leave as is.