Overheating issue with the old slant 6

Uh-oh....Is that red oily stuff floating on top of the water in that bucket? Is that from the engine? Looks like trannie fluid......not good. Rad cap at least looks OK for pressure rating'; I kinda feel the same way about the lever types; I don't use them at all anymore.

Time to stop driving the car; looks like the trannie cooler tube in the bottom of the radiator has sprung a leak. You can use a hose to bypass the cooler lines back to themselves (I would do that like TONIGHT to try to limit water into the trannie), but hopefully there is not much water in the trannie. I would be pulling the trannie pan numero uno job to see if there is any water in the bottom, and any indication in the trannie fluid, like discolored foaming or gunky stuff.

(Rad obviously need to be rebuilt/replaced.)

If the trannie is clean (may or may not be), then a full cooling flush is needed first to get all the oil out of the engine block and cooling system. Use a real flush product and do it 2-3 times. You gotta get all that oil out before you can start re-testing the engine cooling system. After flush, I'd pull all of the hoses off and rub them clean inside, and do a seaprate, soapy clean in the radiator, water pump, themo housing, everything I could get off.

Once there, if the trannie has just a little water in it, I would do multiple flushes with cheap trannie fluid and cheap filters driven for just 1-2 miles between flushes, checking for more water after sitting overnight in the pan each time. Only after I found the pan free of water after sitting overnight, would I put more cheap fluid in it and drive for 10-20 miles, check again to see if there was no water in the pan after sitting overnight, and do it again if I found any. Once clean of water after a longer drive and an overnight sit, then I'd put in good stuff and filter. There may be a better way, but you gotta get the water out; it's cheaper than a rebuild. (Then I would cross my fingers!)