cracked slant block

One suggestion, try Evans Waterless Coolant. I have it in 6 of my 7 vehicles. No more corrosion worries and you can run w/ the radiator cap loose so it doesn't build any pressure. My 1984 M-B had a slight heater core leak, which is a 3 day replacement effort and $400 part. I fixed it by flushing silicone sealer thru, silicate seal (core alone), and now run it w/ the cap loose, and no more leaks or coolant loss. If too cheap for Evans ($43/gal), you can buy Sierra coolant at Ace ($12/gal) which is pure propylene glycol (what original Evans was), so some say "should work". I carry it in the trunk for emergencies. But, you must get all water out of the system, including blow out the heater core.