Timing question

[cam.man67]

Ok, so I've got the new motor in, and the timing was supposedly set, but it wasn't fine-tuned. At idle, off-throttle, and low rpm driving, the engine stumbles and bucks some, very much like an out of time engine would. But when throttle is on, it drives just fine. My question is twofold:

1. I'm thinking this means the timing needs to be advanced, which I've already done to the tune of about 5 degrees. The problem is still persisting, but when I advance it any further, my idle rpm drops. Is this normal?

2. Regarding the two vacuum ports on the carb (Edelbrock 1405), is there any reason that I shouldn't go from the alternating vacuum port to the constant one to run the vacuum advance?