A unibody car needs to be properly supported. How that's done depends on if it's a running car on it's wheels, a bare shell or somewhere in between.

Suspension done? Stock, struts & 4 link or somewhere in between? A chassis car is built to the suspension locations. A stock suspension, full body car is built to support the original locations.

Chrome-Moly, but that requires TIG welding and proper fitting.

You can learn the basics sitting at a table with a MIG running beads. But when you get inside of a car, in all kinds of positions, on a round piece of tube, it's not the same. Learning how to fit, plan, make it symmetrical, fit tight to the body, weld places you can't get to, make it look right, use a tape, plumb line, angle finder and a level is 80% of it.

Have you fabricated anything out of steel before? A race car is not a good beginner project.