I thought NAPCO's were always 4 speeds...
If it is a factory 3 speed SM318 type transmission the after market shifters generally sucked. There was no place the the shifter could directly mount so they came with rinky dink brackets that bolted under the tail housing bolts.

If it were me and I couldn't be convinced to install a more modern 5 speed I'd at least look for a 3 or 4 Saginaw where the shift can mount to the tail housing. Floor shift 3 speed were common in 6 cylinder Camaros.

Lots people seem to be installing s10 type 5 speeds.
I'm not sure of all the details but 5 gears with an overdrive 5th will make your truck more usable on the highway.

If you are using it primarily off road the granny 4 speeds will make you happier.