You want ported vacuum for egr. Ported vacuum is not present at idle but is present shortly after the throttle blade starts to open. Reason for this is that EGR at idle makes it run rough, possibly stall. You also want it to go thru a thermal vacuum switch so that it doesn't apply with the engine cold, same reasons.
Basically, the same source at the distributor vacuum advance.
don't forget egr usually has a delay relay along with the thermo vac switch.
myself I remove all of the OEM crap when going to a edelbrock carb. I only use the dist vac advance on the ported side of eddy carb after installing a set of the MrGasket mopar mech advance springs, rear 3/8 vac/nipple to power brake booster, front 3/8 vac nipple to pcv valve.
I also suggest marking off the damper timing marks so you can set the timing @ 32* BTDC after recurving the dist mech advance.
I also suggest recalibrating the 1405 my 440 needed larger jets/needles/step up springs 2 stage rich on primary and 4 stages on secondary lighter step up spring (I have a big lumpy cam though) and the 600cfm feeds it just fine.
those 2 things woke my 78 MH 440 up on the street.