Steel, absolutely.

You can heat-treat it yourself .. a ghetto solution works fine.
Put up a fire, chuck it in, after 30 mins in coals, cover with sand, take out next day, done.

Even nasa did the big Apollo 11 parts like this, back in the day.

You can easily sand to about 0.02 mm accuracy, using a large wide belt sander.
You really want the best most powerful sander you can loan or rent for a day, about 40 grit belts.

Mark out where you want the material to come off, by using a sharpie crosswise.
Where the marks have come off you went about 0.03 - 0.04 mm deep.

It is very fast, but noisy and unpleasant.
I did 2.2 m long ways like this, in 20x50x2200 mm tool steel flats, and 30x80x100, welded / bolted to substrates.

A ground straight edge of some type is necessary, strong light behind, you can easily see 0.01 mm errors.
My vertex 1000 mm straightedge (woodworking, surprisingly) is good enough, guaranteed to 0.04 mm over 1 m. About 100€ ebay.
The machinist ground straightedge is more accurate, but only 40 cm long.

It takes only about half an hour to do one surface to about 0.01 - 0.02 mm.
Edges do get rounded, so only the center is pretty flat.

I use a Festool 105 sander, best in the world, 1200W, but it is very very expensive. (Noisy). Use earmuffs.
Just rent the biggest possible wide flat belt sander, maybe 30$ for one day.

If the beams are bent, they are easily straightened with some 24 mm all-thread and 80x80 tool steel flats drilled through, used as a press.
I did no heat treating, simply sanded straight, since the tool steel flats are almost straight already.

Look at how metal boats are welded, online.
Tacks, widely spaced, opposite corners, bit at a time. Do not weld all-in from one end at first - this will bend it.