Fine tuning a 350 SBC

Hi guys! I'm quite new here and I've had some very good and useful answers from you lads, Thanks a lot!

My Problem:
I have a Chevy G20 with a rebuilt 350 SBC with Edelbrock Performer intake and 600 cfm performer carb on it. It has been left standing for about 3 years, so I decided to reset it. I have a HEI dizzy on it, and it ran well with about 30° BTDC!!! I thought what the hell, it ran nicely. BUT: I've checked the mileage and its about 8 MPG, thats not so good. And then after about 100 miles of running quite nicely, it stopped. I've chekced all the plugs, on one the inner electrode is burnt away to nothing, the others have signs of carbon deposits, showing a too rich mixture, would never have guessed. So I checked the whole engine through, found a couple of vacuum leaks and the plug gap was set much too small on all plugs. Would the plug gap make such a difference? Funnily enough it ran beautifully. Cam, crank and Pissers are stock.
I don't have a PCV valve or EGR system or anything connected, but all outlets are plugged. Would connecting a PCV-System make better mileage. Could it be valves set wrong?
I have heard, that you can set 8 valves at TDC for piston no1 if you have a stock cam. and the other 8 valves at TDC on exhaust stroke. Is that true? is it exact?
Please help me out, I'm a bit concerned and slowly falling out of grace with my towing-buddies ;-) THANX
Madmax