I need to replace the hard line that runs across the front to the soft lines that go to the calipers.

What size should be used on a '51 3100? ...or is stock size ok?

My dual MC came with one 20" piece of 3/16" brake line and one 60" piece of 1/4" brake line which will run from the MC to the split that runs to the wheels. ...it was labeled "universal".