I believe the split is 1968. 68 and prior slant 6 engines had a smaller crankshaft register (hole where the converter hub fits) than the 69 and forward. So, if you swap in a later engine in front of the early transmission, you will need an adapter ring to slip into the crankshaft register to properly center the converter. Otherwise, you can and will damage the converter bushing and front seal at the very least and might damage the front pump and converter itself. I think @Charrlie_S has the correct size adapter rings. At least he did for a long time.
