What year is the truck?
Given the different positioning of alt and a/c compressor on 94/95 and 96+ there might be slight differences in overall length for a given year, as Will alluded to.
The rule of thumb is a 1" shorter belt.
I just did the vac pump delete last week and bought the AC Delco heavy duty kevlar belt from Rock Auto, part # K061010HD, oem # 19186309.
the 06 stands for 6 grooves, and 1010 means 101.00" inches. So look up the stock belt number. For my '95 it's K061020, and so far the 1010 works just fine for me.