I would definitely advocate a fab shop. Stainless if you’re road driver. You can make the dimensions a little taller and leverage the rear cross member lip for support. A cradle will be better than the ply and watch out for the hand brake articulation on a car that age and its proximity to the rear of the tank.
I’d ask for them to fit two unions for fuelling. You’ll only need one for now on a carb car… but if you ever want to fit injection or a new motor later you’ll have the second port for fuel return… a few pence now can save hundreds later.
You’ll likely have the Old Ford lever style fuel sender (assuming it still works)… this would be an ideal time to consider a more modern and reliable set up … it just means a slightly different fuel sender bolt pattern.
Finally don’t forget to ask for a drain plug if you want one. The one I recently ordered from Mulfab was supplied without one. Not a major issue but could be convenient for ease of drain down in the future.
If you want to take a look at some considerations - check out Episode 3 and 4 of my YouTube series… there’s a whole overhaul and replacement of an old rusty fuel tank…. Which I’m kicking myself for not replacing when I first restored the car.