Car won't run past 60mph

Pedal to the metal should get more than 4k rpm. My best guess... First your original slant 6 throttle cable isn't getting WOT. Second, Wrong throttle pressure linkage and/or adjustment has damaged the trans. You said you disconnected it too which was a bad move. I could go into how a tiny plastic bushing cracked, fell out, made about 1/8 inch lost motion in throttle pressure cable, and killed 1000s of Ford transmissions but I regress.
A torque converter OEM for a much heavier vehicle is wrong for your passenger car too.
Without knowing the cars history ( maybe a previous owner put a different rear under it ) a 67 slant six B'cuda would have a 7.25 rear w/276 gear.