I cant get it to post.
Open google and put in s475 vs t76 look at the images.
S475 is massive compared to the MPT76. Support bracket will be manditory for it.
For wastegates, its mainly down to build quality. Heard of a lot of people using cheap 60mm Ebay gates and working. But have heard of my share of failures as well. Dont tend to hear about failures on quality gates. Good gates would be, Tial, Turbonetics, JGS, turbosmart, HKS.
I still just run a Tial 38mm on my 410cu inch with a GT4202 T4 flange. I can run 8 psi no issues. I would think a 44-50mm would be just fine.