Good deal, I know '78s and my non-original '75 are different, but here is how I have the cap vented. Filtered air comes in from the red filter ('75s didn't have the tube through the firewall) and the air filter housing is the low vacuum source to vent the cap. Ported vacuum from the carb base runs the primary vacuum advance and my secondary is capped.
I think the later models like yours had a VSV connected to the intake vacuum that controlled distributor venting, but not familiar with that system or how it would look once de-smogged.
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