Ideally you want an ac servo drive, however I'm thinking the cost would be prohibitive.
If you had a comparator circuit to compare the voltage from the pedal with the voltage from the linear actuator pot you could then use the o/p of the comparator to control your for/rev relay, and to keep the motor from continually going forwards and backwards you'd need some kind of deadband control, maybe a differential amp from both pots feeding another comparator which would give you on/off relay control with adjustable deadband.