Congrats on the purchase!
For me, I like a little rake in 69s. I like the height of the car to have the rear tire top even with the top of qtr panel lip. And the top of front tire just a little below the fender lip. When measuring the bottom of the rocker panel, the rear would be around 1 inch higher than the front.
The only way to change how the car sits is to work with the springs. For my 69 vert, I used Moog 6320 springs. Same specs as a z/28 6308 coil spring, but has a bit more height for the heavier convertible. I had stock mono leafs re-arched 1.5" over stock height. This put my camaro just where I wanted it.
For your car, I agree start at the front, determine what spring height/coil diam. you have. Then move on to the rear.
You said the front springs are new, measure the coil diameter & free height to see determine the spec, i.e. you might have big block springs in a small block car. If they are just too tall then cutting a little bit will lower the front. But if the spring is too soft, you could bottom out. Small adjustments are key.
Then once you have the front where you want it, then work on the rear. You new rear leafs might have been ordered with a drop. If so, you can get them re-arched to the height you want. Otherwise you can order new springs at the height you are looking for.
Yes, changing out all the springs is a bit of work, but it will pay for the look and handling of the car. Shame the PO did not put in the work and just grabbed random springs to install.
