A little out of date on these things. Have a 79 Z I am repairing things on for a relative. As far as known, everything is stock. At 55 mph engine rpms are about 3000 - that sound about right on a 3 speed auto? Trans does not appear to be slipping any so just curious.
Also, does the TH350 have engine braking in all 3 forward gears? I can't remember as I am so used to coasting freely in high.

 

Yes & yes.