Are What RPMS should a /6 be at while driving 55?

Rumblefish is right. Rolling diameter/circumference of the tire and gear ratio will determine how fast the engine must run at a predetermined speed. A 205/70-14 tire on a 2.76:1 rear gear (typical stock ratio for a 73 auto) would require 2200 rpm to achieve 60 mph assuming no slippage anywhere. This does not account for drag either.