Re: Is .060 overbore too much for a 428?
It depends on the price, power level, use, and how much risk you want to take. .060 is a lot for a 428 but not necessarily "too much".
I gather from your post that it will see moderate use, so if you keep the engine temp down (below 200F, pref 180F-190F) you should be fine down to a wall of .090", maybe even .085". That doesn't mean your risk is zero, since a flaw in the cast iron could cause a wall to crack, there is always a small chance of that even in a standard bore.
Turning the engine higher (6000-7000) or making lots of torque (500-600 ftlb) would raise your risk too.
Because of core shift, I don't know of any sure way to get the wall thickness without a carefully calibrated sonic test.
If the engine had run like this for many years I'd say your risk is lower. But, you don't know its history. If it was just bored again, it might be just about to crack, or it might be just fine.
If it's a high price tag, can you make a written deal that you will buy it, sonic test it, and if any of the wall thicknesses come back less than say .090" you will get a refund?
Well that is a messy answer but it's a messy situation too.