You will need to determine what axle length you currently have to start.
When trying to fit the largest wheel possible you cannot rely on measurements from a different vehicle. There are too many manufacturing tolerances that can stack up and what works on on vehicle won't work on another identical vehicle.
You will need to mock up the tire you wish to run using a wheel fitment tool. They can be bought, rented, borrowed, and if your handy, home built using metal or wood.
Once you know exactly how the tire fits in the space you can then determine the best offset of the wheel t fit the space. Then you can determine how much you need the axle shortened.
I recommend looking at the Mark Williams website for more information. And if you need more information give them a call as they have always been very helpful.
https://www.markwilliams.com/tech-index.html
https://1320wheels-com.3dcartstores....Kit_p_147.html
https://www.rsracing.com/TechWheel.aspx#fitting
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Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help.

RIP Bob Parks.
1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo
If it breaks I didn't want it in the first place
The WMB repair thread
http://67-72chevytrucks.com/vboard/s...d.php?t=698377