Rear sub frame -recommended rust proofing

I'm heading towards dropping my rear subframe. Apart from replacing all the bolts that I have mangled, I plan to rust proof the frame. Question is what is the best method? And what is it likely to cost.

Originally I planned go wire brush, paint on some rust converter, prime it and paint it then waxoyl the box sections. (plan to treat the body tub too while the frame is out). I have thought about powder coating or galvanised dip but concerned about distortion and of costs. I want go have it looking good and lasting. Ideas and possible costs appreciated

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