How fast will it go?

by your track times, i would guess that your stock setup was about 110 hp to the wheels, and your new setup is probably 15-20 more. i think that your improvement in times is due mostly to your converter and gear and taller tire. while it takes more horses to turn the rear end, with that converter your launching much higher into the powerband so your getting much more torque at launch.
is this turbo system going to be carbureted? i would recommend not due to tuning issues, and if you go with an easier-to-tune drawthrough setup, you can't intercool it. you can with a blow-thru but the tuning is a nightmare. best bet is to adapt to an EFI setup, its VERY tunable and you can run a big intercooler and bigger turbo. as for hp, the sky's the limit provided your bottom end is strong enough. i think a full-race prepped, properly turbo-ed /6 could easily top 700hp.
just my $.02 but i could talk forced induction all day! love that stuff, especially when its NOT on a japanese rice-banger motor.
-tim