reality is it doesn't matter if your using 3/8 or 1/2 in you should still have the surfaces milled. you will easily have enough heat into the flanges that they may not be straight when your done. with 1/2" you MIGHT get away with not having to if you have it bolted down to a steel bench when you do it.
i would say to do 1/2" id have to get $75/set for them. with my setup i wouldn't be able to plasma cut the mounting holes, they would have to be laid out and drilled.
not to mention i do not have 1/2" on hand so lead time would be longer.