I have a 1950 ford truck that we show here locally in N.W. Ohio. It has a 5.0 340 horsepower GT-40 crate engine in it. The transmission is a Jasper rebuild that was a junk yard find. the only problem that we have encountered is that when coming to a stop, the dirct drive doesn't seem to want to disengage. It upshifts flawsessly and the fluid is red as the day I put it in 2 years ago. The truck operates fine in all other reapects and we have taken it on 2 and 3 hundred mile trips with no problems. Any ideas will be appreaciated.
Thanks
Chuck

High engine idle and throttle pressure can cause this problem.

What are the upshift speeds?

With light throttle 1-2 around 10 mph 2-3 30-35 mph and OD around 45-50

GOing to guess governor sticky, or shift valve sticky.

Perhaps the shift linkage is a little out of adjustment?