[Mr V]

Personally my favorite tool for cutting tubing is a horizontal band saw. You never have to be anywhere near the blade while it is running and they include a vice so that the material is securely held in place.

As far as figuring out where to cut I'd start by using one of the versa gussets lined up with a couple of rivets to create a straight line that is squared to the piece in theory. You can then scribe a line to follow. Now you just need to figure out how to place that line in relation to the blade to get the desired length. You could also line the edge of the blade up with the edge of the gusset so that the cut will be flush with it.

I do like the idea of a jig that would be attached to the saw that would allow you to drop the piece of versa frame over a couple of pins to line things up exactly. Of course you'll also need some way to line up that jig with the saw in case you want to use the saw for other things too.