I would say that your 3 speed bell is a single pattern.
The cast iron slant 4 speed bell (64-66) would be your best choice.
Your 67-68 a body trans should bolt right up to this cast iron bell.
The aluminum OD slant bell uses the large input retainer and will require some effort to bolt the 67-68 a body 4 speed trans. You would need a special adapter ring around the retainer to make up the difference.
I would call Brewers and ask if they have the adapter ring you need.
There were quite a few input retainer diameters, so measure what you have and after you find the bell housing.
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