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...borrow or buy a vernier or dial caliper for your measurements. The plastic one they sell at Harpor Freight for 4 bucks will work fine.
Use this spoke calculator for your length calculations.
Spoke length calculator for wheel building You should print out your results sheets, and put them on a clipboard while building the wheels.
I assume that you are already aware that a decimal 26" rim, being smaller than the 26 x 1 3/8 original, will change the brake reach required for rim contact. If using rim caliper brakes, you'll need to get some longer reach brakes somewhere along the line.
Here is a Raleigh 3 speed conversion using 26" decimal mountain bike rims:
Sorry, I can't help you on your washer question.