Since you have slicks, I'd venture to guess a high 12 sec - low 13 quarter mile in the LX. The 3.55 gears would probably the deal breaker for a 12, if anything. It'll haul though. Which aluminum GT40's are they?
There's nothing wrong with a 3.55. In fact, that's what I have every intention of putting in my heavy 95. For the mods, you'd probably see more benefit from a 3.73.