Yes, I makes sense to say you need a 4 1/2” lift in the back. I can’t recall the details Deaver needs to determine the ride height for their springs, I do remember they wanted front/back weight and I took some measurements from various points, that being said, fender wells are not a good reference point as the openings are different front/back. I would get specific details from Alcan on what measurements they need, if any, or the best way to determine what spring lift is needed.
But….your best point of reference would be from ground to pinch weld, park on a level surface(bring a level), measure ground to pinch welds behind front tires and in front of rear tires, take the difference and add the height of your current rear lift blocks, that should get you the number needed.
Report back, I’ll be doing this sometime this summer to have Alcan make me a new set to replace my tired old Deavers. Good luck!