The only difference between A-body /6 and SB v8 Z-bar from 67-76 is the length of the tube. SB is a 7.06" tube, /6 is 8.00".
IF you cut 15/16" off the BELLHOUSING end of the /6 tube, you will now have a SB Z-bar. No other differences.
We do make a new one if you prefer: 1967-76 A-BODY SMALL...
Couple more drawings for reference. Looks like you should be able to use the input as-is with this style bearing: PILOT ROLLER BEARING/ADAPTER - Mopar A833 4-Speed Transmission & Component Specialists
Here are all the tailhousings for reference. Assuming you don't want to change the tail / main shaft....you might end making a custom bracket to move the shifter forward.
If you are stripping the threads off, I would say there are definitely some geometry issues. Can you post a few pics of what your dealing with? /6 through BB/Hemi's all used the same threaded rod design. Some are different lengths, offsets, etc...but same thread / nut size.
My educated guess is that they are all 3 neutral. I would have a shop spin them to know for sure. It's a lot of work to put a car together, and find out otherwise.
Your car uses the 122 tooth wheel with 10.625" bolt pattern on the pressure plate. The diaphragm style pressure plates we have use a 10.550" bolt pattern. You can either ADD that pattern to your existing wheel, or the new Ram steel wheels have both 10.550" and 10.625" bolt patterns.
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Like a few others have mentioned, all you need to do is put the correct bearing retainer on your 1:1 unit. ALL 64-74 A-body 4-speeds used a 307 front bearing with a 3.70" bolt circle, so this retainer would bolt on and fit your O/D bell with the 5.125" bore diameter.
23 SPLINE BILLET 5.125...