Is 51 degrees the coldest it got?, it should get colder than that. R12 (if it is) should get down to about 38 at the vent, if it working correctly What do you see when you look at the site glass?

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed