Miss/Stumble & Low vac

I'm buying another one. 79 F-150 4WD 351 M 6" lift, only 1 prob. low Vac and slight stumble/Miss. Let me give some back ground that I know of. This engine is clean the guy I'm buying it from has installed, New; RV Comp Cam, Adj. Hyd Roller Rockers, Timing chain, Heads, Dist, Plugs, Wires, Volt Reg., Batt. And a junk yard intake looks nice (But I know Looks can be decieving) to top it all off an old looking Holley 2bbl. He did a Vac test and said was only pulling 9" I dont know but that sounds awfully low, shouldn't it be in the 30's, now would that cause the miss/ stumble and what would actually cause the low Vacuum. Thanks in advance for any Thoughts:cheers: R.J.