I am going to take calipers off to be powder coated, is it perfectly fine to leave the car up on 4 jacks using the pinch weld points for an extended period of time?
I wouldn't leave it on the pinch welds at all, I wouldn't even jack it from that point. If you go about 6 inches inboard from the pinch weld there is a boxed frame rail that is much stronger than the pinch weld is. I would jack it up from there and place the jack stands there for storage and you can leave it like that long term if you wish.
I just installed Steeda jacking rails on my 2018 GT and they are well made, install easily, and allow you to lift the entire car with one jack. Once the car is lifted, simply rest it down on a pair if jack stands and all is good.