Converted almost 100 R12 systems to 134a with the 35.00 kits with no mods other than what the kit provides. And with no long-term failures due to conversion either!
Sure you can be political correct and flush the system, install orings yadda yadda yadda... Just do it the olde fashion way.
Step 1: fix the leak if there is one.
Step 2: install the retro-fit 134a fittings
Step 3: Evacuate, install oil charge provided and charge the system.
Step 4: enjoy cold A/C
As far as discharge temps, I have found no difference noticeable. I had put over 70K miles after a retro-fit on a 93 Dodge van with no failures. And My 74 280 works great down here in Florida after conversion
The Spiderman is Right