Richard, I need your email address as I've sent you 3 P/Ms with no response.  Thanks, Bob [svopwr@gmail.com]

Holley list 6647. Holley book has this as a Mercury Marine 454. Fine, no CFM flow rating but jetted 68/primary, 70/sec... power valves [8.5s] in both primary and secondary. Other internet info. found has this as a 600 cfm. Wrong, has down leg boosters, "172" stamped throttle blades [pri and sec] and all indications and measurements point to a 750/780 cfm, vacuum secondaries, just like a 3310.  No "J" tube bowl vents [marine application] and a oddball throttle linkage lever, not the usual GM throttle lever found on a gazillion Holley/GM carbs. What gives? Did Mercury Marine rate the airflow different? Now I know that Mercury Marine had their own way of doing things as the Rochester Q-Jet for their marine engines are much different from the GM automotive Q-Jet as nothing from a typical rebuilding kit fits.  That kit is exclusivly M/M available from the dealers only. Found this out the hard way. BTW M/M Q-jet rebuilding kits are about $250.00 and that is jobber price.   

Jay, have you ever run into problems with cyl. head drain holes and head bolt interference due to the headbolt blocking the drain? Perhaps a careless drilling of the drain holes that were not offset enough? If so what was your solution to the problem?