You need to figure out what timing chain and water pump combo to run...
The 68 originally took the old style timing chain cover and cast iron water pump. Th,e timing marks for this are on the pass side of the engine. In 70 they changed the timing marks over to the driver's side and ran the aluminum water pump and the lower radaitor outlet changed sides also. If you mis-match the timing chain cover and water pumps, they can block the timing marks...
Figure out What side you need the timing marks on and what style water pump you are going to use and set it up for that.
Use the original harmonic balancer, and if the mark is in the wrong position, find TDC with a piston stop tool, then mark TDC to go with the timing marks on the timing cover.