The early trucks were designed mostly in conjunction with International Harvester. They built lots within the first years ('50- on), as well as Mack and Diamond T. Later came Kaiser Jeep and AM general. All the gassers had the excellent Continental R-6602. A massive 602 CI six and a 5-speed straight transmission by Spicer, reportedly the largest 5-speed synchromesh ever built for road use.
For a short time, they were built by KJ with the Mack ENDT-673 Diesel engine and an OD transmission. Many gassers underwent conversions by the military to Diesel with this powerplant. It was given the "A1" suffix to indicate a Mack-powered truck.
Starting around 1968 they were built with the LDT-465-1A multifuel engine and 5-speed straight transmission, and given the "A2" designator.
With the 800-series, all of them I've seen were by KJ or AM General. They had the distinctive front sheetmetal to accomodate the Cummins NHC-250 Diesel, again with the OD trans.
The 900's were either AM General or General Products Division. With International buying up the leavings of AM General from GM now (they will reportedly be building Hummers too), I'm not sure what brand things will be.
dg
