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it is likely your AC hoses are still the original 50-year-old hoses.
I believe the common belief is even in like-new condition, hoses designed for R12 are less than optimal for 134A, as the newer refrigerant is molecularly smaller and subject to leaking if used in R-12 hose.
While unlikely, you should check to see if your hoses are marked as “barrier” hoses, indicating their previous replacement. If that turns out to be the case then you have no issue with your hoses.
otherwise, you should consider replacing all of your hoses.
if you come to that conclusion, I highly urge you to look into the Aeroquip EZ-clip system, as suggested some years ago by John Taphorn. I used that system when reassembling 2511 and it requires no special crimping tools and is easily done at home.
additionally, the fittings on the original York compressor hoses are not compatible with a new 134a Sanden compressor. The old fittings are a flare design, and the newer fittings are different and incorporate rubber O-rings.
I successfully installed a newer parallel flow condenser, fan and shroud unit purchased from vintage air. This is much easier than trying to find an original 50 year old unit, and size-for-size the parallel flow design is more efficient than the old tube and fin design
you may find in your research there are many who have just purged the R12, flushed the system, and used 134A in a totally vintage Pantera system. Your research should also show you that this is not optimal or advised by those that truly understand automotive AC.
if this is sounding like too much of a project for your talents, you might want to seek out an independent shop with experience in such conversions for advice and assistance
not trying to be a Debbie downer, but this upgrade is not as easy as changing to LED bulbs or installing a new set of headers.
Larry