A833 4speed OD trans Swap Question

Right.

The only amount of measuring that may need to be done here with any luck is an output spline tooth count. It doesn't matter which number is had, 30 or 26, as long as both transmissions are the same count and both are an A body, the entire gear set will go from the O/D case with the 5.1" front bearing retainer, into the 1:1 case with the 4.3" retainer.

That way, your idea still works, by having a gear set around. The nice thing about doing a gear swap over an entire trans swap is that it is completely reversible without modifying the bellhousing.