I still think it should have a 55 or 65 powervalve. with vacuum low like that the holes in your primary butterflies could be opened up a couple drill sizes. no bigger than 1/8 inch though. I think they are about 1/16 stock but my carbs always get holes opened up to 3/32 then 1/8 if needed. if the throttle blades can be closed more you can stabilize the idle usually.
And pay good attention to your idle mix screw position. mine always end up out a little more than usually recommended. if they are in too far you could end up with an unstable idle in gear too.

Don.