plenty of power when I rev the engine,but little power when moving 5.0 gt

Does the car roll freely in neutral, meaning the brakes are not dragging. Dragging brakes will tend to stall the torque converter and keep the car from moving forward in gear. If the brakes are not dragging, then the transmissions may have a failed stator one-way clutch in the torque converter or there may be a stuck spool in the valve body causing it to start in second gear rather than first gear. A reputable transmission shop wouldn't touch a transmission if they believed the engine to have a low power condition. The engine must be running properly before the transmission can be properly diagnosed.