Welcome
If you dont plan to do any other work and just wish to spin the blower a little faster for a bit more boost, then personally I wouldn't go too much smaller than an 87mm pulley (often marketed as a "10psi pulley"). With everything in proper working order a stock L67 should see approx 5-6psi. The factory Eaton pulley is a very tight fit and requires a special puller to remove it. I'd recommend contacting Steve at Mace Engineering or Mark at TTT Engineering who are both site sponsors and can supply this pulley and also a loan of the pulley removal tool.
A boost gauge can be fitted a few ways, but the easiest without requiring any other mods would probably be to read it direct from the lower inlet manifold (LIM). If you look closely at the LIM just under the blower snout you'll see a small black rubber cap about 8mm round sticking out about 30mm. This rubber cap is blocking a metal fitting which you can hook up your gauge to to read the psi pushing through it.
Similarly, an air/fuel mix gauge can be fitted without any dramas. Just fit it as recommended by the brand of gauge you buy.
Hope that helps,
Cheers