Ever wipe a cam out ? same symptoms---strange only 4 of ehaust push rods. Checking your timing gear set.
400 68 GTO I had---nylon timing gear set ,severe carb back fire ,guess what??
one of my SBC's severe carb back fire---cam wiped out ,a few bent push rods.
Both spit up raw fuel through the carb. Both I shut down ASAP with minimal damage. On the chevy, dropped the pan checking for shrapnal, minimal ,most caught by the filter ,changed the cam push rods etc ,ran it hard after that for about 10 years. Blocks now at home in the Anglia ,rotating assembly boxed up.
Really what else could it be but bad cam timing ? Only the upward force of the piston would be strong enough to bind those pushrods and big blocks are ,er ,BIG. Let us know--steve.