Have a compression test and leakdown test done on the engine, that will let you know what kind of shape it's really in.
Other than that and assuming the engine checks out good, you wouldn't be the first guy to install a blower on a high mileage motor. I would try to stay real close to 400rwhp if you have any real desire to not grenade the engine. 450rwhp, even with a really conservative tune and regardless of mileage is a ticking time bomb without forged internals, imo.
Your existing mods.... Well, you're gonna lose the plenum with the addition of the KB, so I wouldn't worry about it. In the interest of keeping your current engine alive for a while, figuring you'll be intercooled as well which is pretty much mandatory with screw blowers, I would have it set up to push 8 or 9 lbs with a somewhat conservative tune and I think that should get you right at if not a little over 400rwhp.
Putting this or any blower on will increase the rate at which the engine wears, in terms of maintaining combustion chamber compression, not to mention additional stress on rods and bearings; but again, with a conservative tune and not driving like an ******* constantly your engine should live on for a while.
Get this KB strapped on there, then make a plan to start saving for a built shortblock and some heads. After you acquire those, feel free to turn the boost up and tune a little more aggressively and you'll have a real monster
2001 GT Convertible
8 cylinders, 5 gears, red.
6 cylinders, 3 gears, also red.
Remember my motto kids: Be bitter, stay angry and blame everyone. It's on my fridge magnet!