Originally Posted by
thumpism
I had a yellow one. The original rims were the Schwinn 26 x 1 3/8 S-6 (597mm) and might be hard to find. You could try "normal" 26 x 1 3/8 (590mm) instead of the 559mm rims on there now. The rear drum would work with anything but one of the larger sizes would be best for use with a front caliper like you have on there.
I was curious so I went looking. Yes, the Deluxe Twinn tandems of the late '70's had 26 x 1 3/8 S-6 Schwinn tires. Hard to find now. A 700C (aka 29-inch) rim is only 1 inch larger in diameter so a conversion to 700 x 38C tires is viable if the tires fit.
FWIW: I think the original spokes were "105 gauge" aka 12 gauge. The Twinns were heavy, awkward beasts when I moved them around the shop.