I am not sure what year you ar looking for but the older trucks tend to be a lot narrower than new trucks. I used a GMC4500 ( same as isuzu npr) for my truck. Here is my build pictures. I use it for a tow pig for our camper.
http://rides.webshots.com/album/580587250vqfuun


Shane
1959 Apache 3100 Fleetside
1959 Apache 3200 NAPCO
1957 Chevy 5700 Low Cab Forward
1959 GMC F370 LCF
Another 1959 Apache 3100 Fleetside
1959 Chevy 5700 LCF
1946 Chevy 1/2 ton
1958 GMC 600
1974 Elcamino SS