Maybe I missed your point; is it strong enough already? I would say the block will be weakened with sleeves, but the sleeved cylinders will be fine. So, how thick do you want the sleeves? With the thin spots a proper sleeve will go through. A thin sleeve will weaken the old cylinder where it is thin so it can crack, then the whole culinder will be deformed. If you go thick: This means it will be a half wet - half dry sleeve, and coolant can get in between. Could be some issues there. When this happens to me I machine away the whole cylinder and add a thick sleeve, preferably with a small flange on top. A thick sleeve means appx 0.2" thick. This is less than your good cylinders, and the support from the remains is not worth much when they are not all around the circle.
I would say that sleeves can be added anytime, but the original block is at your hand now. And, for racing you will always have some maintenance dismantling, checking etc., so the workload would not matter too much if you need to redo it. Try it as it is! My