R-12 and R134a are not incompatible.  The issue is that the oils they use are.  Not evacuating the system is asking for trouble. 

IMO, the old systems worked well with R12.  They can be converted, but there are two issues.  First, the old style condensors really aren't designed for R134a, which needs a different design to properly reject the additional heat the R134a carries.  At a minimum I would swap to a 6-pass condensor so you don't have to deal with a system that gets cool, but not cold.  This isn't an issue for everyone, and I can't say why, but some people complain about it, others don't.  I would just swap it to avoid a potential issue. 

The second is the compressor, which, while it will work fine on R134a, is a dinosaur.  If you want to keep it original then fine, but the old compressors eat about 20HP in operation.  The Sandens use like 4.  You also will not have belt issues with a Sanden when converting to R134a. 

If it were me, I would convert to a Sanden, install barrier hoses, and swap to a 6-pass condensor if you are going to convert it to R134a.  If it works and just needs a top off then go ahead and top it off with R12. 

I agree with switching to an axial compressor like the sanden.  Do the right thing and have the old R-12 reclaimed by a licensed A/C tech.  To do the job properly, you're basically just going to reuse the evaporator (after it's flushed) and replace everything else.  There are also replacement evaporator coils that are more efficient which can also be retrofitted (A/C tech has progressed since the 1960s).  You may be better off just going with a complete kit.