Broken porcelain insulators is usually a result of impact damage (unlikely unless you dropped something like a small nut in there) or detonation. With the valve guides as badly worn as they were, you were probably running over lean from air sucking down the guides, causing serious detonation on the highway. You won't hear this, unlike spark knock at slow speeds. Overheats the insulators, so they shatter when you suddenly open the throttle and dump a bunch of cold air on them! Check the unbroken ones for signs of blistering. If they are OK, it was a vac leak at the affected cylinders.
Check the fuel mixture when you get the heads back on,and check for vac leaks.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!