Sounds like you are pretty well covered. But since I only recently figured this out myself, I wanted to share what I learned.

I second the advice that a lean mix idles okay, but tends to choke and backfire under load. That confused the heck out of me for quite a while, but I finally understand it now.

A quick test (if nobody mentioned it) is to pull your choke when it starts to stumble and die and see if the system picks up and runs well.

The mixture adjustment is very deceiving. My SUs had to be set much richer than I thought correct before they would behave properly. The procedure to check your mixture is really simple and probably covered elsewhere.

At idle, take a flat-blade screwdriver and lift the dashpots the width of the screwdriver blade. If the motor increases idle and stays fast, or if it immediately start to stumble, then you are either too rich or too lean. If it increases idle briefly then drops below normal idle, you are right about correct. Be sure you re-balance the carbs after you set idle and be sure that the linkages move both carbs in unison. I figure you probably read this somewhere else, but this works for me quite well on my HIF6s (when the car will run).

Later SUs (at least my HIF6s) have little "lift rods" on the backside that allow you to raise the dashpots to check idle. If you have them, they will be little spring-loaded wire things, probably on the body on the in-between side of the carbs. That is, on the side that is closest to the neighboring carb - just below the dashpot.

Hope it keeps going for you.