Practice on and or adjust your welder for 1/4" plate.
With everything nice and clean I'd use 3/32 or 1/8" 7018 whichever works best for you and your welder in practice. Some years ago I welded perches on a Dana axle this way.

Back then I thought it needed lots of weld... Today I think just one good clean pass on each side will do the trick (same as factory perches were welded on).
The more weld and more passes the more likely to distort the tube/housing because you're only welding on the top side.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-