I don't know for sure, but I think the 1ton should have the R151 tranny?
It should be listed on the dataplate. Pop open the hood, should be dead center on the top of the firewall. Pre '88 anyway. In 88 they moved it to the door jamb.
Those 1ton 2wd trucks are great! They have either 3.90, 4.10, or a few even had 4.30 gears in them.
Is it a single or dually rear?
You mentioned a dumper? so it's a small dump truck then?
Post a pic if you can!

1986 Pickup extracab longbed "Woody edition"
SAS - 33's, 5.29's, lockright rear.

02 IS300 SportCross - my "DD"
94 2wd v6 extracab
04 Outback Wagon- wife's/family car