Glad it is going well.
In the spirit of completely over thinking this.......
Do some volume calculations on the protrusion before riveting and the shop head after riveting. Even though it is difficult to account for the material lost due to the rivet swelling in the hole, it is apparent that 1.5 diameters protrusion is actually on the long side (at least for these fairly short rivets we are using). I kept coming up with something closer to 1.3. Some research in the old rivet manuals referenced 1.3 diameters protrusion.
It has been a while since I have done any deep thinking on rivet length, but this is what I remember.
Those 6.5's were the hot ticket for me, but I'm sure a 6 or 7 would be fine.