42RH in 68 Dart advice/request

Yeah if the guy doesn't know the difference between an RH or RE series transmission, he shouldn't give any advice, let alone touch your car. lol
There are some places that sells a solenoid for the lock-up portion of the transmission.
A500 Lock-Up Overdrive Solenoid
Also, If I'm not mistaken I think it was @Abodybomber who rigged up a dual toggle switch for his auto 4 speed conversion 1 for OD and the other for Lock-up