Aluminum links? Who would do something so stupid like that? Oh yeah...me.
You'll want 7075, at least 1.75" diameter, make sure there's a good bit of meat left around the threads after drilling and tapping.. I wouldn't feel comfortable with less than .25" - meaning a 1.25" thread heim should go in nothing smaller than 1.75 rod.
1.75" would be about 25% stronger in bending than DOM, and 2" would be ~90% stronger.
That said, it will bend almost 3x more easily than steel.. it just doesn't STAY bent. it's a weird feeling the first few times you land on the link.
I'm running 2" lowers with a 1.25" shank forged johnny joints, and 1.75" uppers with 1" shank forged johnny joints. no regrets.
As for fatigue endurance.. you won't see it in this lifetime.