[Oblarg]

Originally Posted by AdamHeard

We generally square up our frames with clamps or squares before riveting.

This is the practice that I've found works best, too, both with riveting and bolting. It's far easier to allow some slop in your hole placement/sizing and clamp it in the right position before tightening than it is to try to machine everything to a perfect fit - in fact, I've found the latter to be effectively impossible if you're stuck with just a drill press, and still surprisingly difficult even with a good mill.

Edit: Also, if you decide that you want tighter holes to prevent the frame from getting out of square after taking some abuse, match-drilling through your gussets once you've clamped the whole assembly is pretty foolproof.