[dsraven]

another thing that can work well to more accurately hold the tape is to use a small plastic C clamp to hold it in place on the frame. that way you can get a more accurate measure because the tape doesn't move.
if having trouble with tape sticking to the floor you can wire brush a spot, blow the dust off and mark the floor with a shot of paint. when it dries it will hold a mark better in that it doesn't stick to your shoe if you step on it and won't come off the floor if it gets wet etc.
one thing that is pretty important is to get the frame sitting nice and solid on some stands. solid enough so it won't move when the weight of the frame is removed when cutting off the front and also won't move when sliding the new clip back on and lining things up. I welded on some cross braces in an X pattern when my frame was apart. then ground it off after everything was done and cross members were in place. I used jack stands with the sliding part removed and a piece of square tube welded on top. the tube had holes for some large threaded rod to go through vertically and also had nuts welded on the square tube to accept threaded rod. the threaded rod could be locked down to the tube with jam nuts. the rod went vertically and supported the frame. then, where the rods connected to the frame above the stands , I welded some 3/8 bolts to the end of the larger vertical rod so the 3/8 bolts could go through some existing holes in the frame. I placed one of these units at each corner and was able to adjust the frame for level and rake angle. they really make the adjustment accurate and also hold the frame sturdy and solid. of course periodic checking is advised especially after a large unit is removed or added or the frame gets bumped hard enough to make it questionable. always do your measurements for level at the same place so you know you are comparing apples to apples. I recommend a digital level, even a small one placed on top of a long level. you would be surprised at how far off the eyeball bubble levels can be even if they look dead on. then you place a digital level on and see how far out it is or can be. also, always use the level in the same direction in case the level is out a bit. a good test is to place it on something level, then turn it 180 deg and see if it is still reading level