I would consider what HRL suggested regarding motor mounts. Not that they are loose but where they are located. I have seen some conversions that are using the "bird leg" front mounts and a rear mount for the TH350 transmission. With this configuration there is an extreme distance between the mounts and can cause the engine to torque over to one side which may cause the throttle linkage to bind up.
If you are using the "center" mount position on the engine with the rear mount it is likely OK and would look for another cause.
Fred
