Id change the expansion valve also , and flush the system completely . The compressor will have to be removed and drained of all old oil and then flushed and re-filled with R-134 compatible oil . Once all that is done you should be ok . Old compressors are 50/50 with death from R-134 so your semi- informed .
I had the conversion done to an Olds I used to own and it lasted a week. You really do need to evacuate the system before you refill/convert. The kit will come with the oil and fittings you will need. If you don't have the machine, find someone with one to charge the system for you rather than guessing, could make a big difference. As for the conversion (besides the charge) almost anyone should be able to do the work...like I suggested, make sure all the old stuff is out of the system though.
Hey, you might wanna check out TNN or the new spike or whatever and they just had a show about it last week. It was on two guys garage. I do remember him saying not to use the cheap conversion kits because its overkill for you compressor. They did it to a Chevy nova i believe and they had to replace the expansion valve and they did something to the compressor.
I've since heard that you have to evacuate all the old oil and freon or it will mix poorly and cause nasty things to happen. Also, if it is currently inop for another reason, it could muck the system too (ie clogged lines). thanks, I'll look up TNN.