Correct tire pressure for B/F Goodrich T/a drag radials?

Tire pressure all depends on tire contact patch area and weight of vehicle.

For the Mustang, Ford uses a 245 series tire and recommends 30psi.

Now if you go wider, you have a larger surface area so you can decrease pressure to your tires. How much depends on the surface area of the new tires.

You can calculate it mathematially if you really cared that much, but just experiment. Since you didn't give a tire size, i'll just say start at 30psi and experiment, if you have 315's you might be able to drop down to 20-25psi