converter change

The crank snout got larger ~1968, both in the slant and small block engines. You can get an adapter ring to go from early converter to later engine, but not vice-versa.

You must keep rotating and wiggling the converter until it goes all the way in. I think the 2 tabs on the front pump drive are what must align to allow this. When all in, the snout should be at least 1/2" back from the front of the case. The manual shows holding a straight edge across to verify. If you don't do so, it won't bolt up correctly to the engine, and you risk destroying the pump if you force it. Tranny rebuilders install a strap to keep the converter seated, and also to support its weight when shipping. Don't remove until the tranny is almost installed.

I don't know if slant and V-8 converters interchange.