Brake line sizing

[Skip Fix]

Plumbing a dual MC on the 64 Impala project and using a manual setup(409 is so wide it is tough to pull a valve cover with a booster). Previously it had a 283 and a aftermarket 9" booster and Vette MC with the 4 wheel drums. I will put discs on it but for now want it to be able to stop in the shop rolling it around. Noticed the factory main line is 1/4" then at both front wheels and where it Ts on the passenger to go to the rear drops down to 3/16. Both my Camaro and Firebirds have 3/16 front lines but 1/4 to the rear and were disc/drum cars. Why did they use 1/4" line to the rear drums and this old one used 3/16? And when I use front discs use 3/16 from master to each front and a 1/4 to the rear? Factory line to the rear runs inside the frame which is not Kosher in the bellhousing area for NHRA so probably will re route that one than also.