heres is how i do this on rusty blocks i have bore enough of them to get some idea what can clean up if pits are filled with rust then it can be abit hard to see them .so then you just have to bore them. i bore them under the the bore size by. 005 so if lets say i going to .040 i stop at .035 .i look for the bore with the worst pitts . but i like to true deck the block first .so your looking at alot of money to see if its going to clean up i mag thru some rust .but if this thing has to be rusted very bad you can not mag ?if so then you better walk away from the block it can be a can of worms were bores start poping up rust pits and mains are all pitted . if the block is that rusty it can be bake and blasted. you need alot of evap arust stuff not cheap been there done it .a hot tank with a hot mix will eat most rust off the block then wire wheel is how i do them its alot of work . you want the block clean first all rust off then try to do all finsh machine work last