1970's Gas Pipe Schwinns had a 13/16" seat tube ID.

Try finding a nice seat post for that. They can be found, but they are horribly cheap steel with horrible finishes. If extended very tall, they bend. You still need a wonky shim or something to make a modern 7/8" seat clamp to work.

So, I bought a ~six foot length of 7/8" solid rod 6061-t6 for $14.00 and cut a piece to my desired seat post length. Chucked it in a lathe (a friend actually) and took it down to 13/16" except for the exact amount at the top required to fill the saddle clamp. Plus the exposed little taper to prevent stress risers. I left that exposed for inspection convenience. Sanded and polished.

I have used the leftover rod to make seat posts for two other bikes. CHEAP, pretty, and STRONG.