FWIW I've had good luck with 134. I dumped a can of 134 on top of 12 once, and it worked GREAT!! I've converted systems with a fitting before. No flush, nothing. Theyve worked fine.

As long as you have no leaks in the system, and can get high pressure, and low pressure, you're good to go.

87 Raider 4D56td v5MT1
31's..Basically Stock