Corvette Forums - Corvette Enthusiast Site - Looking At Purchasing A 1999 Corvette

Vettes are easy and inexpensive to modify.
Your 99 LS1 M6 had a 345 HP factory rating. It will dyno at around 295 RWHP. the car can do the 1/4 in, lets say, 13.4 at 105.

Modifications I'd do are below. I would creep up on the power you want a stage at a time, in this order, until you have enough:
LS6 intake manifold - these are the factory manifold from the 405 HP rated 2002-2004 ZO6 and add about 4-5 RWHP. There is some slight EGR relocationr equired on your car but there are kits. Good thing about this is that you can usually find these at very low cost - takes offs from people going to really expensive aftermarket manifolds. I sold mine for $100.
Air box: take your pick among them, Blackwing and MTI work well, many people swear by the Halltek although its expensive. Adds another 4-5 RWHP although many people will tell you it adds more (I've never seen it on the dyno).
These two mods together will show up consistently on the dyno at about 10 RWHP extra plus added torque at nearly all RPM.
Long tube headers. There are many choices. Kooks are well regarded and known to fit well. But there are others. You get what you pay for and what you pay for is a good fit, and long tubes -- the longer the better. Given the nature of your engine (only lightly modified), get 1 3/4 inch diameter and 2 3/4 to 3 inch collectors, no bigger. These will add 12-18 RWHP.
At the same time, you really should add a good H-pipe (get that from the header manufacturer) and a cat back exhaust: a lot of people like Borla but Corsa or Magnaflo are good, too. GHL is great but costs big bucks and is only worth it if you have serious mods. the H pipe and cat back won't add a lot - maybe up to another 10 RWHP.
AT this point, you need to have the car tuned: the engine computer re-programmed, because the engine is running considerably more powerful now. Find a tuner, someone with a dyno and LS1Edit programmer and experience. Make a deal with him up front for the price of this tune AND future tunes, because you will be back many times. I paid $500 for the first tune/dyno session with an understanding the next time back for adjustments was free, and $150 per session everytime thereafter.
Tuned, this combination of intake and exhaust should net about 325-335 RWHP compared to the stock 295. 1/4 at about 13 - 13.1. Note you have yet to go into the engine.
You can put on drag radials. I think 305s will fit. This might get you into the high 12s right there.
At this point, there are several routes you can take:
Cheap, effective, dead end for the money you spend: install a set of higher ratio rocker arms (1.8:1). These follow the stock cam profile but open the valves more. Its work any backyard mechanic can do easily-not like changing a cam. And it adds about 12-20 RWHP and you can definately feel it. Have it returned. You now have about 335-345 RWHP total.
Add a cam. Problem is without head work they don't do a whole lot. But . . . add a good cam, call a manufacturer you trust and tell them (call one of the sponsors of this site) what you got, take their advice. Have it re-tuned. This will add maybe 20-30 RWHP versus the 12-20- the high ratio lifters got you. You now have about 350 RWHP, maybe 360 if you have a really right-on-the-edge cam. You should be in the high 12s consistently with drag radials, probably not with stock street tires.
You can add a heads a cam kit. Prices vary and you get what you pay for, but any reasonable one, such as the SLP kit, will, after tuning, put you at over 375 RWHP, maybe up to 415, and in the mid 12s.