An 80 is a big axle... but very strong.
Ford or Chevy.... the ford D60 front is high pinion, but not as strong as chevy from what i'm informed.
Personally i'd go chevy, which is right hand drop and then upgrade to a set of 35 spline outters, change the inners and joints if you wish as well.
Why atlas ? What's wrong with something like an np205 - will cost you £1500 less and can be made just as strong. Although the low ratio is only 2:1
My upcoming build is TH400, NP205, NP203 doubler, D80 rear diff, 14b front diff, 47 spline front and rear. Rear inners are only 40 spline so i can use an ARB, front has a spool. I purposely didn't go for an atlas but will run the 205 with a 34 spline titan shaft + other small upgrades inside.
G