What's wrong with the axle that's in your '55? Looks like it should be fine if you go through and check kingpins, and tie rod ends. If it's all OK, I'd use it.
The only thing I don't care for on stock Chevy axles is the tie rod diameter. They're really too small and can flex if one wheel hits something. I replace them with a 1.25" tie rod, and switch the steering arms to allow the use of heim ends on the tie rod. Makes a nice solid setup with no chance of death wobble.
I am sure if done right a straight axle is fine. Just like adding power steering results vary . A car with aftermarket control arms and a 500 box drives better than one with stock control arms and a 605.
Your 55 is a great find if I was as lucky as you I would mechanically do less than more to keep it like you found it but just my opinion and everyone has one of those. No question the mechanical fuel injection is not a streetable option and those cafeteria race seats might have to go as well. Have fun with it!
Friend of mine has a 10 second gasser with an Econoline mono beam front axle and drum brakes all around. Scary to stop but drives great
No reason an Econoline axle with drum brakes shouldn't stop extremely well! I put an Econoline axle in my '63 Falcon gasser with a BBC, and it would lock up all 4 wheels without power assist. Used a '68 Mustang dual drum master with it.
I put all new drums, and brake shoes in it when I installed it. Even at high drag speed drag strip runs it would haul the car to a stop quickly.
The only issue with the axle that is in is that it is the narrower one. I prefer it to be a little wider, but more importantly, for the springs to be parallel.
When the frame gets redone, I'd rather have it wider.
Then the Chevy truck '55-'59, or Greebriar van axle would be a better choice. Both are just a little wider, and look better in a Tri 5 to me. Those mid 60's Greenbriar van axles are pretty tough to find though. But they come 5 x 4.75" bolt pattern, so they're a great choice.
I love it! It's a really nice find. My take on this car is to address the questionable engineering making it safe. If you are not going to drag race it, then making it safe for street driving would be my goal. If major engineering is in mind then maybe another 55 build would be better. This car is a 60s gasser in its present state. As I said that is my take, but it is your car. What I can't figure out is how 17 spline axles held up under clutch drops and violent shifts, when most early gassers used a Pontiac or Olds rear axle. Anyway I love the car.