Your first step is to tear it down and see what happened. My guess is that you got too much anti-freeze in the oil and that caused bearing failure, but that is just a guess. The fact that 5&6 never had much compression could be that the rings never seated or the valve seats were too wide. On a re-ring without boring, you want to use nothing but cast iron rings. The milky oil is a sign of coolant. Quite a few blocks cracked behind the push rods and leaked coolant into the oil. If you have another engine, you may not need to spend a whole lot. I drove 270's when they were new and they were built to work all the time. A 302 is noticeably more power and a lot of your 270 stuff will work on a 302. A 350 is doable, but it requires making room for it and a lot of other things.
