WHAT IS THIS??

Hi all,

I will start off by saying i have 1970 Dodge Dart Swinger 2 door. 313/904. I have been lurking and gathering info for a while now. i have made purchases on recommendations as well as found other advise that pertains to my Dart. i purchased from another FABO user, when i got the car home we had a freeze that night and i busted a freeze plug as the PO had just filled with water after a water pump replacement. Figured i would replace all them, so i have remove the engine sent the trans to a shop for rebuild as it wasn't working properly, and since i had it out......when i was able to get to the freeze plugs i could see several were very rusty, 2 were replaced with rubber compression plugs, and another was weeping water ever so slightly. so i knocked them out and found this, what could i do to clean out the passages to make sure all of this sludge and what feels like sand out of engine? this is a picture of what came out of the freeze plug in trans area on the left. i have dug out as much as i could and then i used a wire to break loose the stuff that was up and around the plug. all plugs have been removed and i wanted opinions on what i should do next?? any help would be appreciated.

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Casting sand in the bottom of the cooling jackets. Pull all the plugs out and get the garden hose out. It will take a long time to flush it all out. Straighten a coat hanger to get in and loosen the stuff up. You will be shocked at how much you get out of it.