I am retrofitting my '74 Charger with factory air to R134. I have two cans in the system in the car, and it seems to blow cool at idle. I haven't taken it on the road to see if it will blow colder when moving.

I am concerned about adding too much 134, as I've read that only 60-80% of R-12 capacity is required before efficiency starts to drop. So how many cans of 134 should this system take? Anyone have any experiences or thoughts? Thanks!