Suggestion and Questions on new build

[RKO]

Need suggestions and answers

Building a 66 short wide box frame off
What I have done so far is rebuilt and added the front suspension from a 86 C10. I used the factory sway bar. I didn't use a power steering bracket. Instead I cut a section from the 86 and welded it on the inside of the frame and then cut out the indent needed for the steering box. I I wanted to lower the front 1 inch so I think I can get by with lowering with using air bags
I used the rear end from the same 86 C10. I want to lower the rear 4 inches. So I used two inch lower blocks and air bags on rear. The 86 had a rear sway bar so I'm using that. Didn't like any of the tracking arms on the market so I built my own on the top of the rear end. Along with I built new shock mounts to get the shocks more straight up and down
I cut the center cross member drive shaft hole and made it taller to clear drive shaft.
I'm using a mild built 350 with a late 700R4 transmission
I moved the motor mounts ahead two inches and used the rear transmissions mount from the 86
Just got the Cab back on today no other sheet metal yet

Question one will 275R75X15 tires on factory 86 8inch wide ralley wheels clear the fenders and box sides?

Next question I want to cruise at 75 MPH. What would be a good rear end gears to use?

I built my own rear fuel tank out of stainless steel. Looks great. But is it DOT approved?
Nebraska says I'm ok in this state. I'm a certified welder. No one else will say yes or no?

I know I need to get pictures