My bet is you have a vacuum leak.
Check all vacume lines, plug off one and note any change in idle. Check intake manifold, sealing rings installed?, re-tighten all bolts. Shoot WD-40 around mating surface/gasket, carb base, throttle shaft on carb and note any change in idle. If it changes, you got a leak.
Carb re-built? Internal air leaks can cause havoc with idle.
If no leaks, retard timing. Be sure Vacuum is plugged off to the advance before setting timing.
Howard's M4F is a good cam. It has a high lift but the overlap is minimal and would not cause the idle problem you have described.
The dual carbs and Fenton headers with the M4F cam will really wake your 261 up! Recommend Holly/Webber carbs from Stovebolt Engine Co. for dual set-up!
