Lay a straight edge across the tips of the valves
Check your carb for missing screws
I'm very curious to see what the plugs all look like.
Putting air into each cylinder with rockers removed will be very telling.
A thorough diagnosis will make for a viable and, install with confidence, repair!
Any chance of it being over revved when you downshifted?
Input or countershaft of trans may be locked up?
Hope for the best, be prepared for the worst
