390/45R15
They also offer it in a 17" if you prefer.
On the 17" I've gone 1.67 60ft on a full weight two wheel drive quad cab Dodge 2500. The only suspension mods were cal tracs and a couple of leafs removed in the rear. I'm sure guys have probably gone faster. That's just my experience.
That's about as much tire as you can find in a drag radial.
Though your fabricator may be able to set your suspension and chassis to better utilize a smaller footprint tire.
But, on your rig with the look you are going after, I think it would be a good choice for form and function.