[March 2, 2017: Original Post and I did not have it working yet]
[March 26, 2017: Edited with Updates and now I have it working]
My initial excitement for this modification has been tempered with incorrect fitment of the given design for my 2015 Taz 4 and all the problems I have ran into. But after replacing an incorrectly sized rail and modifying other miscellaneous details I have it working and I am very happy.
To start with I had to purchase a second rail and cut it again. As you can see in the original photo, the rail wasn't long enough. My rail was cut within 1/2mm to the length defined in the instructions (500mm - 64mm = 436mm); but as you can see, there is not enough length for the T-Nuts to hold it in place. It was even worse than the photo showed, as I had not yet squared the mounts on the ends, which added almost another 5mm to how short the rail was. The new rail I cut at 442mm in length and I felt it was still short, but workable. I believe 446mm would have been the perfect length, 10mm longer than the instructions list.
A better approach is to mount the brackets without a rail, square as best as you can, then measure the distance on the inside, subtract a few mm for adjustments; and then cut your rail to this length. For me this would have been near 446mm.
Many of the bolts listed a size that were too short to provide enough threads to reach the nut through the part; but not all. This could be due to slight irregular printing of the parts; but such tolerances I generally expect to be included in the design. But also going up to the next standard bolt length may make them too long if used with a T-NUT (as they will them hit the inside of the rail before making the T-Nut tight) or a closed end heat insert nut (hit the inside plastic on the other side of the heat insert) . So be prepared to sand/drill bolt head locations, cut a longer bolt down by a few mm, or use some extra washers to get the right size.
Most of the recessed head cutouts where too tight. I am not talking about the bolt thread area, just were the head is inset. The socket heads were very hard to press in (again back to print tolerances taken into consideration for the design). I often had to drill out the recessed head cutouts or at least work a bolt in and out to get it go in without undo force. Adding 0.5mm to the diameter of those cut outs in design would have solved this.
While the design includes a printed T-Bracket, the instructions do not mention using it. It seems he never put one together with the printed T-Bracket as it doesn't fit without modification. The tolerance on the edge of the T was not great enough for it reach minimum X position, so some shaving was required. Not a big deal but annoying. The belt studs also required omitting the extra nut; otherwise it put the belt WAY out of the plane causing binding at both ends. Even with the carriage washer still present it causes the belt to be slightly out of plane, but removing it is not required.
The Zip ties for fastening the belt may also bind with the bearings at Max X if you don't make them very tight against the stud. Getting the belt loop smaller and zip ties closer are a requirement; but again I wish the design included a little greater tolerance as getting them very tight was difficult with big fingers.
Its fully functional now; and it is a BIG improvement over the stock setup. I could twist the head on the stock system with very light touch and not so with this modification. Further with the stock rails, the print head would droop by 0.5mm minimum in the center versus the right and left edges of the print bed. Now it is rock solid and straight; and I swear it runs more quiet then the stock system!