the 134a adapter connectors screw over top of the old R12 connectors. I would at least pressurize the system and and see if it leaks out in a few days.
Assuming you have no leaks continue on
remove the compressor dump the oil, flush that, remove the filter drier receiver dump oil & flush it,
put it back in because you're going to be taking it out again. (Or not)
Add proper amount of pag oil, some refrigerant short out the low pressure and run it a while then take it all apart and flush all again! ( Or not)
I think I would also include replacing the orifice tube, they're pretty inexpensive.
Through all this you should be able to learn how to use the gauges, vac pump, and basic properties of how refrigerant works from liquid to gas, gas to liquid, how to read the gauges how to read the tp charts which are the same as the gauges.
All the above is just my suggestion it's not written in stone it's your truck your decisions and any free advice is worth what you paid for it
Yes people do the conversion don't take anything apart flush the oil run it and flush it again. Hopefully the system won't suffer from Black Death