Just a suggestion, start with a large bolt pattern Axle. 8-3/4 Sure grip 789 case rear with 3.23-3.55 gears if your going to drive this car. Upgrade the rear springs, shocks, and these cars ride and handle very well with radial tires. I also added a set of 340 torsion bars to mine. I see you already have disc brakes, thats a nice touch.
As for a motor, the sky is the limit. Again you can build a nice 340,360, or build a stroker 408 all pump gas with a mild hydraulic cam, aluminum intake, carb and headers. And you will have a nice fun car with enough power to burn the tires and be able to merge on the interstate.
At this point you will need to change the sending unit in the gas tank to a 3/8 fuel line pickup. And run a new 3/8 line to the front of the car. That line with a good mechanical fuel pump on the motor will supply 500 HP easy trouble free.
Mancini has the fuel sender, and a nice electronic ignition conversion kits, i got all my stuff from them.
Again make a list of things you want then come back on here and ask for ways to achieve them, lots of good advice on this site. Old school or new school.