Have you tried to stick a screwdriver or something down inside the front and rear carbs to push against the throttle blades to see if they are seated? Or disconnect the linkage so that you are on the primary only?
Something to consider, seems to me taht motor idled about 450rpmish and the original carbs' internal passages are sized so that a lower depression will flow fuel for the idle circuit. Are you using a carb from say the mid 60's for your primary? That would be more consistant with the higher idle than the original rochester. I am not sure how that would play out (using the original) but it might be messing with you in odd ways.