Generally, I've heard that longtubes will cut your ground clearance a bit, but it seems that ANY aftermarket exhaust will do that, really. Oddly enough, my buddy's '90 notch with longtubes and off-road H-pipe has MORE ground clearance than my '89 notch with stock shorties and an off-road X-pipe ... but then, he's running stock springs while I'm running Eibach Drag springs (which I think only lower the car just a teeny-tiny bit, but still...). Adding lowering springs to the equation would pretty much guarantee you're going to hate speed bumps and stuff.
If this is your DD, I dunno if I'd recommend messing with LT's. Myself, I'd sooner worry about banging up a cheap off-road H- or X-pipe that I can replace in an hour or so, versus banging up a set of pricey longtubes that are a major PITA to change out. That, and consider the fact that LT's really don't gain you all that much of an increase in power (especially considering the cost and install woes), you'll probably have heat soak problems with your starter, and LT's actually QUIET your exhaust down. (My aforementioned buddy's notch has the exact same mufflers as I do, and they're WAY quiet and mellow compared to my setup.)
Just some things to consider before clicking that "buy now" button ... unless it's already too late...