Oh, the work I go through for people on this forum...I've got 5 of the 5 1/4" gaskets (never install them). I emailed you one...let me know if you get it.
I checked the listings, and the same gasket fits all years. I even checked one against my '68 casting (larger internal bearing starting in ''68), and it matches. I don't know where you got the oddball nor what it fits. If you've got gasket paper, go to work, as thickness won't matter on this, unless it's
really thick.
Good Luck,
Dan