Re: Problem w/Coolant system - Head Gaskets?
For whatever reasons, it sounds like a classic low-water boil-over. When running, the water temp got up pretty high, but not boiling. when shut off, all cooling stops other than radiation from the block. So what water was there started to boil, with the trapped and growing steam helping do a "big vomit". I could hazard a guess that your temp gauge may not have read trouble, if it was out of the solid water area. Or broke/marginal sensor/gauge. But I would lean more towards the water level was WAY down (not good for tranny either, if the only cooler you have is the one in the radiator tank!).
Now, if that is really what happened, why? Is it possible that when replacing A/F you left it air-bound, without a full-fill of A/F, even though it "appeared" full? That could trap a slug of air in the upper hoses/block. All my 302's I had a Prestone Flush'N''Fill tee in the return heater hose, and I could get real close to a fill that way (by burping air out the tee with the tee cap off). Even then, I would still have to add later after some run/cooloff cycles. I always looked in the radiator for a while, then felt I could trust the coolant recovery tank level. A leak in/at ends of recovery hose, or vacuum leak on radiator cap can fool you into thinking everything is OK 'cuz the recovery tank is OK (but radiator could be down, and can't suck A/F back into radiator when cooling down due to a leak under radiator vacuum!).
I Have seen brown in radiators as a sealant, came in new cars for years, would build up crud on sides of recovery tank. Was a favorite in garages after major engine work. But the brown was a build-up, the overall A/F color was stil green. Not saying what you see can't be oil, will be interesting what the crankcase shows... low on oil, loaded with oil and water mix, or OK! (I'm pullin' for the big OK for ya).
Let us know what you find out, and anybody else feel free to jump in...