R12 Conversion to R134A?

[1977_LEROY]

I know my compressor is good and original but not sure if system was converted to R134A... the high and low pressure fittings look newer than original vehicle. I just put a belt on the AC compressor and it runs. I know the system has some pressure on it but it does not get cool when I turn on AC. Should I look for someone to just add some R12 to the system and see if the AC will work or do I need to just convert the system over to R134A? If so, what is the best way, economical way to do this? Or is it best to just put R12 in and do it next winter once it cools off. Any advice is greatly appreciated.