Countersunk bolts are self-aligning. So when you bolt that adapter plate to the bell housing it's going to bolt into the same position every time, theoretically. If it were a free floating setup such as you would have with hex head bolts then there would be opportunity for it to move around. I would Mount up the adapter plate check the run out in accordance with the service manual and adjust it for Center, torque down those bolts make sure everything still in spec and then drill a couple of alignment holes and run a pin through them so that that plate goes on to the same spot every time in the future. Not that you'd have a lot of call for removing it. If it's off, it would not surprise me if the later model offset dial pin that was mentioned would also work on our flatheads to adjust it. The factory does all of that when they built the assembly so theoretically if your adapter plate is perfectly centered on the bell housing, then you should not be any more off than the factory had it. Yeah, some of us can be belt and suspender types. But inherently, we're lazy, LOL. We'd rather do it once, do it right, and never have to do it again. The thing about the adapter plate is that those guys don't know your specific situation. Maybe you swapped out the bell housing, and even the factory will tell you if you do that you need to double check the run out.