My '84 Xtra cab has a 19 gal tank.  

 I believe those tanks only came in the long wheel base trucks because the mounting provisions simply don't fit in the short wheel base trucks. The tank may be too long but if you could get it to fit a short wheel base rig It won't fit all the way under the cab and may sit directly over the rear axle which may get in the way during suspension compression. Haven't seen it done...but I suppose nothing is impossible.