I can't remember which year the register in the tail of the crank got bigger, but I think it was 68. You can get a set of calipers to measure the inside diameter of the register before you order a converter. Hopefully, I explained it to where it makes sense. The later style transmissions have different spline counts on the input shaft than the 65 does, so that's why you can't simply reuse your existing converter.
Only the slant 6 and the 273 has a small crank hub and they used it until the end of production, LA318 on the other hand only has the large hub. The slant 6 hub upsized in 68 the above statement only applies to the 273.
