I have a Pro Systems 950 HP on my 408/W
( I know, I know BIG CARB ) that,s what he built me when I gave him my spec's.
I had what sounds like the same thing you have. After trying numerous suggestions from helpful members & none solving the problem I put a lot of thought into it & concluded it was not transitioning correctly. Soooooo I started with decrease idle air bleed from .076--.073. Then increased idle feed restrictor from.035---.038. Much better but not quite there, still a very slight lean spot when breathing on the throttle @ cruise.
Then with everything set as good as I could get it I pulled the carb & checked the transition slot, YIKES!!! only .010 exposed.
I set the throttle so there was .020 exposed put it back on with no further adjustments---YAHOO-- it idles a little faster than I would like, but I can live with that because now I have instant response when I breath on the pedal @ cruise 1500-2300rpm.
A/F will run from high 14's low 15's when cruising & drop to mid to high 13's when I even think about increasing throttle.
Hope this is of some help
Craig
I also run with a 02 LM1