I'd run a comaritave compression check to eliminate burnt or bent valves. Check under the distributor cap for debris.I'm sorry but I allways think the worst and hope for the best.I sheard the dowl in the camshaft last year.It can happen ,I got it out ,it wasn't fun. watch how far the valves move. Cams have lobes wear flat. But that wouldn't have been sudden.And if the lifters wore you would have had to adjust out clearence at the rockers. I had two lobes go flat and mushroom the lifters that were riding on them.One was an intake on #1 and exhaust on #7 or #8. The filter caught all debris. I removed the engine and had it gone through. No dammage to crank or bearings.Machinist polished the crank ,replaced bearings and replaced second and
oil rings. I had the gapless top rings.
I had a freind (back in the day )that was accustom to buying new 427 short blocks twice a year, putting his 427 stuff on them.In 66 he tried a new 428short block with a his old used 427 flywheel. He only got to the end of his alley before backing it back to his garrage and removeing the engine.He tried to return it to dealer .They would take it back .He tried to scatter ti and collect the warrasnty.He had it on a trailer made from the front of a car frame.with radiatror and
oil and it would just throw the belts. he loned ti to a guy with a stock car,it blew the clutch and broke the bellhousung. He then sold it and later bought it back from a guy that had taken it to a machine shop and had it bored .040" over.and then lost interest in it.And sold it to me when I bought a 63 1/2 Ford Gallexy that had a old police intercepter 428 with what turned out to be a stuck valve in it.428 takes a 428 flywheel.You can swap almost everything among all the other FEs but not 428 flywheels and 428 engines those go to gether.