At 1st I thought that was just a block pic for reference but if that is the "one" it (the block) for sure needs to be baked clean. W that amt of crud it will unfortunately never be right. I'd also (w a freeze plug/jb weld) plug the large irrregularly shaped large block water passage in the rear of each deck, as said punch out the slits to match the figure eight top passages AND drill an 1/2" (at least) hole in the block/deck/gasket between #3 and #5, and #4 and #6 cyls in the outer edge of the deck adding a completely new water passage. The proper place on this edge there's 2 outer head bolt holes and you want to drill the 1/2" passage "in/up" so that it makes a triangle w the 2 head bolt holes. 2 ex valves are paired at that point and that alot of blocks have NO passage there and it is exactly the spot that needs a passage and a large one at that. And an anti cavitation plate welded on the pump vanes and at least a 1-1 crank/wp pulley ratio. Good luck to you on this dilemma and holler on what you think & how it turns out