Sounds like the problem is that the splines do not extend far enough for the clutch disc to fully disengage. I think there are three possible solutions.

1. Get a clutch disc with a 'shorter' hub, or grind down the hub on your disc so the engine side is 'shorter'

2. Shim the tranny out a little so your clutch disc doesn't foul on the tranny end of the splines.

3. lengthen the splines/clean them up with a cut off wheel or file. You shouldn't need to do too much- 1/8th or 1/4 of an inch maybe? Alot of that is just deepening/widening existing spline grooves becuase they get narrower and shallower towards the end where the clutch disc is sticking/stopping.

One way to tell for sure if this is the problem is to lie under the truck with the clutch cover in a spot that you can see through. Have someone step on the clutch pedal and observe. If there pressure plate is fully disengaged but the clutch disc isn't its probably the splines. You could also put in the tranny with the clutch disc and no pressure plate to see if there are splines behind the clutch disc so that it can disengage.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi