I, for one am very interested in how your machine is going to run using your old button over again. Even AMBAC, the manufacturer of the pump, tells you to recalculate the button thickness if you install a new IP.
Here is a link to a document from AMBAC...
Selecting the Correct Injection Pump Timing Button. Look at the second page near the bottom...a paragraph in RED marked as IMPORTANT..."
Plunger buttons are not included with model 50 pumps or hydraulic heads. The required plunger button must be selected when a new pump, or an old pump with a new head is installed on an engine".
This guy in Texas may have a lot of parts for sale and be able to rebuild and test the pumps, but if he's telling people to re-use their old buttons, he's passing out incorrect information. That's the reason that AMBAC stamps the port closing dimentions on the side of the pumps. The only way you could use your old button over is if he rebuilt a pump for you and put your old hyraulic head in the rebuilt pump.
Also, if your calculations came up with needing a size 17 button, I would think you should go back and recheck your calculations, probably a simple math error.