The electric fan shroud doesn't block radiator like some I have seen that are flat. It allows airflow from across the surface and it has a large amount of CFM, 3300, compared to others I have seen.
The mention of shroud vent flaps is something new to me. Do you mean cut holes into the electric fan shroud and place something homemade on the back side to prevent backflow?
Timing is well advanced with no pinging at all through accel range. I have a HEI distributor with custom timing curve ground into the weights that I saw in High Performance Pontiac mag back in the 90s.
I also recently started running Ethanol Free gas and it's only avail at 90 Octane if that makes a diff.
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