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The only "oil cooler" that should be there is the power steering fluid cooler.....and that doesn't have "coolant" in it, only raw water and the PS Fluid...
If the PS cooler is leaking, and its original, probably best to replace it....less desirable option would be to remove it and take it to a radiator shop for testing/repair.
Yes they have sleeves and sold several to dirt track racers...balanced,flowed,custom cam,sheet metal intake,1050cfm holley,8500rpm= lite weight powerhouse of 500+HP!!!!!I have heard that these 488 model engines were less than successful. It seems the Al block had no sleeves in the cylinder bores.