Zippy, here's a tip that may or may not help. If you run them through the floor, find and mark the rear and forward points. Then you can drill a small hole through the floor for reference. If you use some string stretched between the two points, you can get a pretty straight line marked out. Then you can measure over for the width to get the other line marked. Like it was mentioned above, it is much easier to weld the seam up if it is tight with as little gap as possible, so it's best (IMO) to not cut too much away the first time.