Truck accelerates when in gear

My old truck’s engine starts accelerating when I put it into gear. This is a 1973 Ford F-100, 3 on the column. I just had it in for work (had let it sit for a long time and had noticed a gas leak at the carb so I had it hauled in). The mechanic cleaned the carb (carb kit), fixed the leaks, changed fuel filters, plugs, wires. It seemed okay when I picked it up. The next time I drove it, it started fine, idled fine, but when I put it into gear the engine began accelerating on its own. As I was driving it would do that intermittently and I had to keep kicking the gas pedal to get it to let off. The engine had done that once before when idling before I found the gas leak and before I took it to the mechanic. Does anyone know what is sticking or why? And should I take it back to the same mechanic or look for someone else?

Could be the cable between your pedal and carb is sticking or the linkage on your carb is held up. Next time it happens, don’t touch the pedal but *open the hood and see if you can identify what’s bound up.
Squirt some PB blaster down the cable to keep it freed up.

*(put the car in neutral or park first, of course).

You may want to check the motor mounts. If a motor mount is broken, the engine may shift position and pull the accelerator open.
If you found a mechanic willing to work on carburetors, go back to him.

Should you need someone to rebuild your carb, I highly recommend these guys:

It could be your fast idle cam is sticking, get some spray carb cleaner (these days it may be labeled throttle body cleaner) and spray and work all the linkages on the carb., then work the throttle making sure the throttle plate isn’t binding. Look for a vacuum leak and make sure the carb is bolted down snugly on a good gasket.

Speaking of finding someone willing to work on carbs, a last resort may be an outboard marine repair shop. Carbs are still the name of the game in marine fuel systems and many of these guys are magicians dealing with older units in need of a little TLC. You might find someone who does double duty.