Pressurization HELP!!!

[James Bennon]

We have a 1973 T337G.
Recently overhauled Front engine and had combustion heater checked by C&D Associates- They sent heater back as not repairable--we re-installed intending to heat with Turbo heat only as before engine overhaul when pressurization worked fine.
We have not been able to reach max differential at altitude since. (At 14K cabin is at 10K)
Pressurization switch check as operable.
Have resealed heat ex- changer.
A. Any suggestions as to where to look for problem?
B. Skymaster shop qualified to work on pressurization?

We are located in West Virginia

Jim Bennon