Keep in mind that your R-134 charge should only be about 75 % of the R-12 charge. I've been doing retro-fits for about three years for Ford, and the reason is that R-134 molecules are much smaller and expand under heat at aviolent rate compared to R-12. It would be easy to overcharge the system, and I know from experience that those 'kits' are very general and are supposed to fit many applications. A normal 5.0 Mustang takes 1.75 lbs of R-12 and should work perfectly with 1.25 lbs of R-134.
Also, the accululator should be replaced because the decessant bag (strainer) inside is soaked with R-12 oil, which will not mix with R-134 oil. Therefore rendering the accumulator almost completely useless.
It sounds like your compressor is trying to freeze up due to lack of oil, which should have had no more than 3 oz of new oil installed, or it is overcharging, due to an overcharge of R-134, or a partially blocked orifice tube. I have seen comressors shut an engine down, and even a compressor that seems to be working perfect, if overcharged, will rob up to 30 horsepower! Something to think about! Good luck.