904 vs 999

If it's the original transmission then it's a A904.
In 1982 the A904 was the only choice for trucks with 225 engine.
The 318 had the A999 transmission.
Both were lockup.

The A904 had 4 front clutch disc and the A999 had 5 front clutch disc.
Also they used different valve bodies and kickdown levers, rest of the gear train was the same.

Direct quote from 1982 Dodge truck parts catalog Torque Converter with A904 transmission - Lock Up High Stall #4293051.

Stall speed listed in the 1982 truck service manual is 1800-2100 rpm for the A904 with 225 engine. Of course many factors determine the actual numbers.
It would be nice if we had the PK numbers on the pan rail.