Pinion Angle/Tailshaft Bushing
and through the accelerator pedal.
the gas pedal is not physically connected to anything vibratory except maybe the firewall. If the firewall is vibrating,could it be that your engine torque-strap is too tight?By that rpm the engine should be dead-smooth.
About the only other thing that can send vibration into the chassis that strongly, would be an out of round tire. But that never gets better. Just worse and worse til you can't stand it anymore.lol.
On thing that does get better is a natural body cadence, which usually gets just noticeable at 55, worsening to mid 60s then getting better again. All my Barracudas have done this, no matter what engine,tranny, or rear-end was in it.The worst was actually a slanty/904 with 2.71s. But is was stimulated by the N50-15s humming along back there. 60mph was dreadful. 55 was bearable,65 better and by 70 it was smooth sailing. AFAIK there is nothing you can do about a vibration like that;except not stimulate it.
