Sorry for the delayed response; work was very hectic last week. First, let me apologize for posting this in Shop Talk instead of the Small Block forum; I didn't realize the other forum existed when I made the original post...
When removing the 6 bent pushrods we noted they were all on intake valves. We inspected the remaining pushrods and rockers and they were in good shape. We replaced the pushrods and two scored rockers, reassembled and checked for spring bind. Everything looked good. Next we verified the timing mark on the damper was at TDC. When we pulled this apart last week we noted the plugs were indicating a very rich mixture. So I dropped the primary fuel jets two sizes from a #75s to #73s.
When we cranked the engine ... nothing. No spark. It seems to help to plug back in the MSD pickup wires
. Hit the key again and it fired right up. We put a timing light on it and the timing was sitting at 4-degrees BTDC! After resetting the base timing to 14-degrees the engine was (understandably) taking the fuel a lot better.
So I think you guys nailed it when you suggested it was a combination of fuel and timing causing a backfire in the intake that lifted the intake valves off their seats and allowed the pushrods to jump off.
The car's definitely more responsive now. Thanks for your help with this question!
Mike