If you mill .070 off the block you will have to run a really thick head gasket to keep the piston from hitting the head.

I have a set of KB107 pistons that are .030 over and they are a hair heavier than a stock magnum piston, can't speak for a std bore kb107. I would not run a new piston in a used block with 30K miles on it if the new ring moves higher up the bore than the old ring, there will be a slight distortion that can unseat the ring when cylinder preassure is at its highest.

If I was in your boat I would have it squared, then .035 milled off and run the .039 fel pro and not the .054 that comes in all the kits or you could mill around .030 and run the mr gasket .028 one.