Question about fabricating large frame parts

Hello all,

I am working on a project that is going to be a large frame component for mounting some precision equipment onto a vehicle frame. The frame will be made out of some 3"x2" 1/8th wall aluminum tubing cut and welded into a "L" shaped frame that has a large base with mounting holes and a 90 degree back plate that also has precise holes. I need some ideas on how to align the frame before welding and keeping it in place while welding. The holes on the base and on the back plate need to align with each other so the components will stay aligned. I figured I will make a fixture to hold it in alignment while welding, but I'm not sure how I will true up my fixture to start. The tolerance is plus or minus 0.050" on the holes. The frame is about 2' x 2' on the base and about 2/3rds of that in height.

Fixture is the way and you will need to practice on a few pcs to see how and where they move to. Then adjust fixture so the parts move INTO the proper position. Weld sequence and direction of travels all need to be documented so can be repeated.
Better would be weld it so it pulls into square and then machine the holes.
Hard to visualize what you are wanting to do without some form of sketch or photo.

I would fixture as detailed above, but drill the holes afterwards.