after you turn the boost up...get on the road and do a few high gear stomps to make sure it is not leaner than about 750 on the top end. I have run a bit leaner but I don't think it really gains much.
then check out first/second and try to lean it down to about the same range.
My experience is that boost beats timing every time so I don't mess with timing much. Sometimes I may actually reduce it in top gear if I need to in able to run the boost.
A 275/60-15 generally fits on an 8" wheel if the back spacing is close to 4". That's a 28" tire. The longer patch will give you more bite than a slightly wider tire. It also loads the turbo better and brings it up quicker. I run Nittos because I don't go to the track anymore. M/T's hook a bit better at the track but they wear out a lot quicker because the rubber compound is much softer.
Just remember that on the street, the speedometer will read about 8% slower than your actual speed unless you put a 27 tooth gear in the speedometer drive at the tranny