NGK (1273) BCPR5E-11 is what I've been using for about 20 years now, never a problem. 5/8 hex for better header clearance, larger gap for electronic ignition.
Is that a lockup style converter? If so, could be locking up prematurely. Just a wild guess! Even if not, still could possibly be a transmission problem.
If that is the original engine, it has the wrong timing cover. Timing mark should be on passenger side, water inlet on left. Timing would be way off. If it is a 71 could be either.
Should be available from any auto parts store, ask for worm shaft seal National part # 7188s, or CR #6141. Also available in a kit with all the seals for the box.
Nice thing about them is the balance factor, they can be used on both internally or externally balanced engines. The only bad part I know of is that they are apparently of Chinese origin... I plan on using one myself for the 408 I am building.