Just some thoughts.

The internal parts of a PowerGlide robs a % of HP to the rear wheels so Carter richened the idle circuit and metering to compensate?

Comparing (on paper) the main jet & metering rod:
YF 2100S versus YF 966S (jet area minus rod area times 100,000)
Main jet 0.0945"....0.1015"
Economy 0.067"....0.073" (356....399)
Midrange 0.060"....0.059" (426....544)
Power 0.050"....0.050" (512....621)

Using Ethanol gas, the YF-966S could be the better choice for a 235.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."