How do you determine "ride height" to make suspension mods??

I found the lowest structural point which was where the front frame rails ran under the cab and met the torsion bar xmember. I sectioned 1" out of that because it was relatively easy and made it nice and flat to tack in a 5" piece of square tubing for the car to sit on.

From there the lowest point was 5" from the ground......well 4 7/8"

You will need a set of tires/wheels for the mock up.....I bought some junk wheels/tires off CL that were close to the height I wanted to run.

A 335 tire is going to need a 12" wide rim. Rims get expensive after 10" wide since cost is a concern. No problem running an 18" rim on the front, stick to about 26" tall tire (35series).