I just replaced my brakes and about half the brake lines on my 54 3100. The sizes where mixed. All the individual lines going to each wheel were 3/16. They appear to be original. The replacement wheel cylinders where for 3/16. The only 1/4 inch line I had was coming off one of the two master cylinder outlets. It ran to a splitter feeding the back wheels and one of the front wheels. Leaving that splitter was 3/16 lines to the individual wheels. The other outlet from the master cylinder was a direct line to my front driver side wheel. It was 3/16. I bought standard coated steel lines from AutoZone. they are straight and come in various lengths with the ends already made up. No need to flare yourself. I was able to get lines that were close enough in length to the originals and bend them myself with no issue. You can get a tubing bender at Harbor Freight for less than $10. If you are careful you could probably bend them without a bender by hand. They were about $5-$6 per section.

Dave from Northern Kentucky
My 54 3100