engine revs at stops

I have a 1956 GMC 100 with an approx 1965 chevy 327 with a "car kit" exhaust emission control system. The stickers says its from Air Quality Products, 1976. Truck is from California. Last week the truck was converted from a 3 speed to a 700r4 transmission.

My problem is that at 15% of stops, the engine revs up and starts spinning the back wheel. My front disc brakes have been able to keep it from moving more than a few inches. I have been shifting it to nuetral to stop the truck. I do not remember the truck having this problem with the old transmission, but I did have my foot on the clutch. I brought it back to transmission shop, but they couldn't figure out what the problem is.

Any help would be appreciated.

 

Is your throttle cable sticking?

 

Throttle linkage is not sticking. On a one occasion I was just idling through rows of parking lot and engine didnt rev up until I stopped in parking space. On freeway, no problem when coasting 5 mph. Engine reved up and tire squeeled when stopped. Takes a few seconds.