I had a new set of rancho springs from a 77 camaro parts car.
I ended up measuring a stock 81 camaro and made them that length. Would have liked to go shorter but wasn't sure if it would effect the suspension. They measured 3 3/4 inches. Worked out OK.
Built a set of caltracs at the same time. What a difference, the rear u joint doesn't try to grind its way thru the tunnel now as the power comes on.