I had a problem with my 58 with a 350,th350 throttle linkage. When stopped at a light and stepping on the brake, the engine would speed up. Taking it out of drive and the engine speed would return to idle. The problem was with the throttle linkage, when stopped at the light the engine torque caused the linkage to open the throttle slightly which caused more torque and more throttle.(Justlike a run away Toyota) This was caused by not having enough play in the linkage. Fixed this by adjusting the linkage longer to make more travel in the accelerator pedal.
You might check linkage when the clutch engages,the engine torque may cause the throttle to open and make it difficult to drive.
