As mentioned earlier there are standard sizes for various models but the important thing is to match the brake line size with the components on the truck. If you are going to use the original rear wheel cylinders and they have 3/16 fittings it wouldn't make any sense to try to adapt 1/4 lines. If you are using a later rear end and it uses a different size line, you will need to change accordingly.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine