I used 3/16" on my '32 chassis and I wish I would have used 1/4 brake line. Its a 1940s style hot rod so 1/4" tubing would have looked more authentic. The people that know look for this kind of stuff. You're building a 1940s AV8 so 1/4" would be the hot ticket.
Modern cars use 3/16" tubing. Maybe back in 1939 1/4" looked safer to the engineers? Later, all cars switched to 3/16" tubing. Some of the bigger trucks from the 1980s I worked on had 1/4" tubing but they had huge wheel cylinders
Here are some of my brake flaring tools. The top box is a Blue Point (Snap-On) Then a Papco and an old K-D double flaring tool. At the phone company garage we had an Imperial double flaring tool that I liked the best. I have never seen another one like it, I keep looking on eBay. That shop was built in 1950 so who knows when they bought that tool. They had a nice little Imperial bender that would make real nice tight bends. I always borrowed it. The benders I have do not make as small of a bend. I need to buy one of those little ones.
I'll show you how I make lines that might be helpful to you. I only work on my personal projects so I don't make brake lines very often. Every once in awhile I made them up at work, when a snow chain crosslink would break and tear a line right off a rear wheel cylinder. That's a different world than building hot rods. Those lines don't need to be pretty, they just need to be safe.
First, make sure your nut is up against the flare before you make your bend. Once you make the bend that nut is not going to slide around it. Tape the nut to the flare, that would be even better. If you shorten the tubing put the nut back on (in the right direction) before you flare it. I got in a hurry at work and made that mistake. I was so proud of my perfect double flare until I looked down and saw the nut laying on the workbench.
Like I said in a previous post. I got bored one day and made a bunch of short pieces of tubing with different degree bends. Probably from 10-degrees on up to 90-degree.
If you look at the bender you will see there are two zeros that are lined up. There is a lot of slop right there there so the zeros can be a lot. You need to make sure they are lined up or your bend is going to be off. In the photo I'm putting a sharpie mark on the tubing right at the end of the bender.
Just to give you an idea. I setup a couple '32 running board brackets to simulate a frame. One that's straight, it might represent a frame side rail and one angled, maybe its a center crossmember. I picked out the two angled test pieces that will work best. You can see the Sharpie marks from when they were setup in the bender.
Lets pretend we bend the real piece of tubing starting where we want that 90 degree bend to be, that's the easy part. Now what is hard is to know where to start that second bend at? If you start the bend just a 1/4" too far the tubing will be too long to fit in there. A 1/4" too short and you could make it work but it won't look as nice. All you need to do to is put your two little test pieces in place and measure the distance between the two sharpie marks. That will tell you where to lineup the tubing with the tubing bender's edge to start your second bend. I've had pretty good luck doing it this way.