Put it overnight in a barrel of molasses. It'll be good as new in the morning. Add some dryer sheets to get rid of the fishy smell.
There was a flathead that sat in the yard at my pal Dave's house, the guys would come over, drink beer, and piss on this old flathead! When I got into my '27 Highboy project, I remembered that poor old engine...it was 1970, It had been out there since about '62. Rolled it over, it was a 59L! Hoisted it into my ex wife's '66 Squareback, brought it home, and soaked it in diesel & WD 40. Days later, I disassembled it, sent it out to Redi-Strip, then down to Ralph's machine shop. Hot tanked and magged, it had NO CRACKS! (one of very few I've ever seen, let alone had) It became my 3/8" X 3/8", found everything between swaps and Paul Schultz, in Santa Cruz. Imagine, we were peeing on it...
I wouldn't disturb it. Obviously some HAMBer is doing a long term corrosion test on that block. Probably used WD40 (fish oil) on it.