Motor Mount help vibration at idle
 

I have 72 6 cylinder 250. I have some pretty bad vibration once I drop it into drive and hold the brake down. there isn't any(nearly as much) vibration in park or when driving down the road. My question is does this seem like the motor mounts are shot? or possible the transmission mounts? I've never changed either but it seems like a pretty straight forward job. Anyone change these and see huge improvements?

Thanks