Welcome sepp. Glad to have you here. How about downloading some pictures of your 55.2 on one of the photo hosting websites and posting a link here. We love old truck pictures.

As others have mentioned we need some more info in order to help diagnose your troubles.
It's a 6-cylinder so probably a 235. Unless the engine has been swapped sometime in the past - and a lot can/does happen in 50+ years. Be sure some of those pics show the motor. Do you have an oil pressure gauge that works? If so what does it read at speed and at idle?
What trans.? 3- or 4-speed?
Is this a 1/2-ton or 3/4-ton?
Is the body solid or does it have considerable rust? Rusted/loose body/cab mounts can add lots of noise.
What kind of tires? Highway tread or mud/snow tread? You said they are balanced? Any shimmy or shake in the steering?
I'm just brain-storming. Maybe none of this helps.
Good luck!

Professional Novice