Thinking about converting from 4speed to 904 or slant six 727.

Yes. A998/A999s come in both slanty and SBM flavors.
IDK what year they came out, probably late 70s/early 80s.


I had an A998 slanty, which had the alternate gear ratios like as come in the A999,
but mine did not have the LU. I swapped the gears into an A904 V8bell, for my 318. The 318 loved it, even tho the ratios were a lil further apart, cuz the 318 has enough torque to pull them with a low-number rear gear. Well to be fair, I installed a 2800 convertor at the same time, Yeehaw!
For you, in the quest for fuel economy, I recommend the A998LU, which usually have ratios of, 2.45-1.45-1.00, splits of .592-.690; but may have the alternate ratios
of, 2.74-1.54-1.00, splits of .562-.649
You can see that, comparatively speaking the 2.45 gearset is slightly tighter, especially on the 2>3 shift. Honestly, the difference, in a low-rpm, lo-po engine is hardly worth searching for. At 3600rpm the 2>3 shift difference is just 148rpm.
So just grab a LU whatever it might be.
If you have a choice, well, I'd pick the standard ratio.
As far as auto-transmissions go, the Torqueflite series are simple and cheap to rebuild. After just a bit of experience, almost anybody can slam the main-box together ..... blindfolded. but save your eyes for the VB, as certain of them have interesting spring and ball variations.