First off...count all yore dawgs...are all of 'em accounted for? None of 'em been layin' around the carb with the air cleaner off? Did the new mini-dobee git sukked in??
Sumthin' went down the gullet inna bad way and scrambled the guts.
Sumthin' fairly large and hard in order to thrash around in the combustion chamber and create a momentary "lockup" between the valve heads and the combustion chamber, that in turn broke the heads off the valves. This is not the same as "dropping a valve" due to gross over-rev.
Sumthin' went down the carb throat on the passenger side carb venturi since it's number five cylinder which is fed from that side of the dual plane intake. Air cleaner retainer nut? What's missing from the hardware on the top of the motor, what were ya doing just before ya cranked the motor?
Are the valve heads still stuck in an exhaust port or in the header for that side? Where'd they go?? They could also be inside the other three runners on the intake manifold fed from the passenger side.
Bummer...this is major rebuild indicator and right here at xmas-time!
Most likely, the rod on #5 is gonna be "bent" also, check that carefully when ya rip into the shortblock.
What's Jeff say, has he seen it?