changing the rear gears (3.07), lowering, and increasing power through some judicious CNC head porting.
best mod might be a LPE TT setup (223-226 mph) but bring around the price of the Z06 to get there!!!
seriously, changing the gearing will help top speed and is probably the cheapest mod. Not sure of the stock Z06 gearing but going a bit "taller" will let that LS6 howl a bit harder in 5th and get you up to the speeds you seek with a bit more power.
getting 450 HP out of the LS6 is pretty easy by updating the air cleaner, throttle body, chip tweak, and potentially a bit of head/cam work. Plastic intake is better than the LS1 so not much to be gained there. I would seek out vette doctors in NY and ask their advice, find a good vette mechanic, have them ship u the parts, and give the pooch a good scare. You don't have far to go to attack the autobaun and will get there faster too!!!!
back in the 60s we had a good old american V8 (460) lincoln. not much in the acceleration dept but just wait till you hit a hill, the porsche guys yelled at us as we croozed by!!!