As for the milage aspect, I'd go fill it up (to the same point you did last time. Be consistant about top-offs after the pump clicks off, etc) and then calculate milage. The gauge is simply too inaccurate to interpolate the way you are trying to do.
Your other issues: You didnt use plat plugs, right?
How old is the coil? How long since a fuel filter? Can you see FP while this is occuring (cowl gauge, etc)?
If you accelerate past 3500 RPM slowly, does it still cut out or does it do better?
More info like that, along with codes, might help the smart folks on here to provide advice that might save a couple of bucks.
Good luck.