1, I'd make stiffer rings if it were me and possible.
2, I prefer mine a little more on axis, but your way "can" work.
3, Trunk mounting is fine.
4, 16awg will be fine
5, Make the box then try it in all different positions before securing it.
6, Welding cable has always worked fine for me, I use the generic 1/0 and it have never let me down.

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