Just a couple of things

I just got our 260Z 2+2 running that we got from a junkyard however I need a timing cover and headers. Right now she sputters and backfires on accel trying to get up the driveway. I suspect the timing is retarded but the brake booster seems weak and that is rebuilt for a 280Z 10 inch diameter. When we fired her up she was at 1400 rpm with steady 17 inches of vacuum I got her down to 1000 than 800 and the vacuum fluctuates between 14 anf 15 slightly at idle. Could my timing be off a tooth or two causing this?