A 1986 Chevy 1-ton dually with a 454 engine most likely had a
TH400 3-speed automatic transmission. However, it's *possible* (though less common) that it could have had a manual transmission, in which case it would likely be a heavy-duty 4-speed.
To be certain, you need to check the vehicle's documentation (owner's manual, build sheet if available) or look at the transmission itself for identification markings.
