help with 12 bolt info

[low69chevy]

Not sure if this is the right section but figured I would post here since alot of you'll run different types of back spacing on your race wheel. I want to order some Moser axles for my 12 bolt figure I'm going to build it and see how long it last with the turbo setup I have planned for it. My question is I'm going to be running 20x12 racelines with 4" b.s. on the street and 15x12 bogarts with 4"b.s. at the strip I just don' know what size axles I need to order what length.
Here's a pik of my truck with the wheels so you can see how much they stick out



This is a pik of a member from a local forum I asked him and he said he nnarrowed it 4.5 inches each side but to me it looks like the wheel is sitting to far inside but not sure his are 17x12 w/4in b.s.