Hi Todd - tough read all run on but think I got it. Up to you to fit compressor with brackets and seems OK just unsure what you meant much longer bolts and is that a problem where they went IN to wherever?
You have your performance but would recheck vent temps. You really can't keep it staying that cold or evaporator WILL ice up - not a maybe. You should notice if any reasonable humidity is doesn't make condensate now then floods ground or floor when you shut it all down.
IDK? Was new expansion valve pre-set or any info with it? Is it adjustable or what?
Low pressures consistent with low temps, strong air flow.
Vacuum to only 26Hg fine if at some altitude otherwise wonder why only that?
R-12 is the liquid gold of course and tolerates a LOT of being off some on charge but suspect expansion valve may have been set for 134a, bulb incorrect location or something.
Whole this is knowing the REAL vent temp and know it's probably as much as five degrees lower right at evaporator is only concern.
It's so old - good for you all around.
Owned a zillion rolling freezers on wheels all OE never touched and didn't keep way too long but tons of miles without hassle just a bit fuelish of course but very dependable cars not part truck and not sure I ever saw a '79 Ranchero? Not the point - cool and a Ford fan some of the time.
Double check vent temps. Infrared thermos for me don't read air temps well or confuse mine. Note if frost seen with a good run or blower speed lowers flow depending on humidity.
BTW - If all is accurate if you held only a 26Hg vacuum even it suggests either wrong reading or you are at close to 4,000 ft. altitude?
It would work it not and probably fine with R-12 but why I just wonder,