Hi,
You say you have a msd dizzy, probably just mechanical with no vacuum advance. If so you need more initial timing, say 16* to start. Set timing then set idle to 800 rpm and see if the stumble is still there. If it's ok you can play with the timing,try for 36* full in by 3,000 rpm.
If it's not timing,put a vacuum gauge on it and see what your idle vacuum is. The power valve in the metering block has to be less than the vacuum. IE: if the vacuum is 9 inches, then the power valve has to be more like 4.5 or no more than 6.
Good luck,
Perry.
