Alex I'd like bolting it together on the ground for $500, please...It isn’t irrelevant if he wants to put it in the car and not have to adjust it after it’s in the car. Measuring beforehand will eliminate all of the working on your back with a drop light/ flash light to make sure this correct.
If he has a lift and can leave the car on it for days waiting for parts to show up, then fine , do it in the car.
I can't be the only one who builds stuff with simple methods here..