Racing slick growth

I have a '66 Dart that has been a race car all its life. It has been HEAVILY abused, but it is probably one of the best performance optioned '66 Dart GTs built. It is also VERY solid, and from 5 feet it looks great. Anywho, The K-frame was cracked in half in two places, the steering box was split, the cowl has cracks running out from the bottom of both pillars, the shocks have been yanked out of the body a couple times, and the lower control arm was bent. Since I am in the process if replacing all the broken parts, and welding up the damage, I turned my attention to the stretched wheel openings. They were starting to blister through the old welds. I hated to start stripping paint, but when I began sanding, I found a different quarter panel has been pieced in there...actually it was done pretty well, but the damage was done in the original paint scheme a long time ago, and it was stick welded in and then the welds were beat down so that the valley could be mudded.
Long story short...I'm replacing the entire quarter and I'm NOT going to stretch the openings. This means the 30 x 10.5 x 15 Goodyear slicks won't be used again. How do you guys measure for slick growth at speed? Things were tight before, but I liked the fit. I'd like to get the tallest tire I can in there.
Any ideas? The car is mini tubbed with inboard A990 Superstock springs and 10 inch wide Supertricks. From inside the trunk it looks close to stock.