/6 rebuild advice
I’m a sucker for slant sixes so I’d stick with it. Here’s a question: are you sure it’s ENGINE oil in the coolant? Not ATF? The internal radiator/trans cooler could have corroded through causing intermingling of coolant and ATF. Just something I thought of to check before you get too crazy into it! A new radiator would solve and may not be a bad idea anyway if it’s old and clogged.