You have a '68 340 right? That originally had the older water pump and timing cover with the timing mark on the cover over on the passenger side. Most (or all) of the newer dampers are for the '70 and up timing marks, which are on the driver's side. What you shows in the ad looks like it is for the later marks on the driver's side.
So 1st thing it to check on which side are the timing marks on the timing cover that you have. If on the driver's side, then you are basically set. If on the passenger side, then you are going to have to make a pointer on the driver's side, or make new marks on the damper to line up with the passenger side timing cover marks.
Either way, with your heads off the engine as the are you are going to have to find EXACT TDC on #1 cylinder. Use a dial indicator with the heads off on #1 piston to find exact TDC, and carefully locate your marks so that zero is EXACT TDC on #1. Since you are going to be messing with cams and/or timing them, you need to be spot on with this to have a solid, accurate reference.
Once you have that zero reference, then you just read the timing on the damper against you zero mark.
It may be confusing a bit, but hang in there; we're all pulling for ya.
