It could be that too, the Holley 1920 was known to be a crapper single barrel, and the Rochester 1 barrel was to be half decent....BOTH are very fickle if not adjusted right Altho, the Holley seems to have the most issues with it, that are known....
A lot of times, the stock settings on a single barrel will greatly differ between engines of the same C.I.D. in a close the same car....Almost most so much as night and day such as this!
Are you going to switch the 408 and the Slant 6 when the 408 is done? OR is that engine going into another car?
I say look at the carburetor a bit closer, FINE tooth comb the thing, see what happens, if your going to keep the slant in the car its currently in, replace that Holley 1920 and put a Rochester in place of it, and see if that doesn't solve some issues! IF you plan to replace the whole motor, then I say get it to run a bit smoother till yer ready to put the 408 in it, and call it good! Put the money into the 408, and carry on....
IF you plan to keep the slant 6 in the car, I say re-do the carburetion with something better....I think the Rochester is the better choice, Carter is no slouch either.....Just hard as it is to believe Holley is to make some really GOOD carburetors but, that 1920 was a troublesome little thing! Change it out to something else and I BET you'd be good to go!