I like under the floor, looks better and looks original, and I think less work.
Good plan on brake line sizes is to use what the vehicle you pulled the brakes from used. In your case a P30 or 3500HD chassis. I believe they were 1/4" from MC to proportioning valve. From there two different 3/16" lines to each front. To the rear a 1/4" and I forget if 3/16" or 1/4" on the axle to each side?
All this you can find by looking at one in the parking lot, or perhaps a friend has one complete you can scope out.
