Slant 6 MPG Upgrades?

You can mitigate that some by slamming the A999 gears into your A904 pb case.

The low first gear ratio of the 999 will not work in the PB case. The splines in the gear train are different then the splines on the output shaft. You would have to use the late output shaft, also. But that creates the problem of compatibility of the new output shaft with the rear punp, and the tailshaft housing bearing/seal.
Plus, if you did use the 999 gear train then a new converter would be required, as the input splines are different, also. But then that would mean a "custom" converter, as the pilot hole in the end of the crank is too small for a standard converter with the correct splines.
I like the PB trans, but if I were looking for the best bang for the buck on fuel economy. I would, in addition to normal engine rebuild procedures.
1) up the CR by milling the head and/or decking the block.
2) 2 or 4 bbl intake manifold (price decides which)
3) I haven't tried any of these myself, but my choice would be, either a Weber progressive 2 bbl, or possibly a spread bore 4 bbl (Q-jet, thermoquad) FI is an option but not cheap, and not TBI (use port FI)
4) Stock exhaust manifold (maybe open up the outlet), and a free flowing single exhaust (2 1/4 - 2 1/2 inch).
5) Of course fine tuning the advance curve, and fuel mixture ( this would depend on what the engine/car likes)
6) While the engine is out I would either open up the torque converter pocket in the crank or use a 68 or later crank, then
7) you can install a later 998/999 style trans with the LU converter
8) remove the 300 lb tool box from the trunk :)