You're going to need motor mounts and bellhousing, so I'll assume straight drive which means flywheel, clutch and starter, all accessory drive brackets up front, fuel pump, maybe rework the wire harness depending on what was done installing the 305.

I'd so some research on the 326 first, not many built those with Aluminum heads, and most aftermarket heads have large 72 cc chambers and 2.11 intake valves unless special made, or they used domed pistons to increase compression.
Still would be a better engine in the 68 than the 305 IMO but it needs some research to find out exactly what's in it. A 350 or 400 may be a better choice in the long run.