Completey Agree with Jeff.
It's all about prep and patience not rushing being the key. You can get very very accurate with shiming and scraping/grinding, Don't rush the welding and keep the runs short to minimise warping trying to balance the weld location not heating one spot too much.

I also think you'll be surprised just how NOT flat or straight some of the Alu profile can be, even the better stuff. . . . .Or should say more expensive stuff.!!
If you want truely truely flat the both would need to be machined surfaced or ground.