I would immediately visit another prop shop or marine machine shop. The literal rule of thumb may well be the 16 is required if you are using a 1-3/8" shaft, however, you and I both know there are safety margins in there to cover everyone. The difference between a 1-1/4" and 1-3/8" bore in a prop just doesn't seem like much to me.
Michigan Wheel, where I bought my new 4-blade props last season, would bore a hole of any size you specified (within reason) as a custom order. As a result of the low volume of 1-3/8" props being sold these days, they didn't have any on the shelf so they considered my order a custom order. It was custom in schedule only, and didn' t cost any more. I just had to wait a bit. No problem. It would be interesting if you called their tech department and asked about the particular size prop, to see if you could get them to provide one for you in the 1-3/8" bore. You would learn the price of a new one, find out if it could be done. It might give you the assurance you need to have your existing props bored out.
It should be relatively straignt forward to string up a wire from your hub to the strut and measure what sort of shaft length you will need. You could also use a dummy wood handrail section, etc. if you could find something to fit through the shaft log (which could be opened up for the test fit. Give yourself a little room to work on the length (see my photo). Of course, if you need more room you can always install a DRIVESAVER later and pick up another inch.
Good luck,
Regards, Paul