To hold the bottom frame tubes exactly where they should be I cut 2 pieces of 1x3" steel wide enough to clear the frame tubes and then some. I scribed a center line then measured where the center of the tubes should be on either side of the center line. these 1x3 tubes measured almost exactly 3 inches and the frame tubes were modeled parallel to the ground and exactly 3" to the bottom of the tubes. So I milled a couple of tabs with a 1-3/8" end mill to make saddles to locate the tubes on top of the 1x3's. In the Bridgeport it was easy to get the depth accurate and scribe a line to line up with the locating lines I had scribed on the 1x3's. once the saddles were tack welded to the 1x3's I measured the distance from the front swing arm pivot and with the large square located the 1x3's on the frame jig. All in all it is very well lined up.