69 Z28 rear springs
 

I'm building a Trans Am type car. I'd like to use parts from Howe racing. The leaf springs thay list come in differnt arch 5 1/4, 6 1/4 and so on. I assume the lower the arch the lower the car. What is the common arch used in most G machines

You'd need to know spring rates for a reference if you want to go by the arch of the spring vs. ride height.

Most companies list ride height as compared to stock. I'd recommend a 3" drop leaf.