The truck dying doing 60 is going to be your biggest clue. Generally this will not be a tune problem. You can drive with points not opening and it will back fire. You can drive with a fouled plug and still drive it. Original coils almost never go out, maybe your old one is still around?
I had one of the arms of my points burn completely off going down the freeway once. I see this is not your problem.
My blue 521 sometimes dies going down the freeway but starts right back up so I can't trouble shoot it. I think I loose all power when it happens as my dummy lights dont come on when it happens but come back on before I can get out of the truck most of the time. Again I see this is not your problem.
Check plug number 4 and 3 for fouling. I had a engine that would foul new plugs after a couple hundred miles. Ended up getting a anti plug fouler for cylinder #4.
If it was running I'd say put a timing light on it to see if all 4 are firing but you have to get it started first to do that.