That's still not going to put them anywhere near a stock 450. 450's have double the 350's power stock, and the power is all in the high rpms where it's usable on the track. The 350's power is much more suited to the trails. Sure, you can spend over a grand and make the suspension as good as a 450, and spend another grand to get the power competitive, but you're still going to be 50 lbs heavier. If your goal is really to race the thing you're better off starting with something that's made for racing.
With my warrior built the way it was, i had nearly $5,000 in aftermarket stuff by the time I was finished with it, plus the $3,000 initial cost, and i still didn't have a shot against the 450's in a real race unless it was against an unexperienced rider that couldn't run it all-out. Granted, i didn't build mine with mx in mind, i'm more into trail riding, and I ended up with an excellent trail machine, but it didn't do well on the track. Then i bought my raptor 700, which totally blows it away on the track, and does almost as good on the trails as the warrior.
I'm not saying this to put down the 350's, i just want to keep people from throwing their life savings at a 350 and then selling it because it wasn't what they hoped it would turn out to be. The 350 definitely isn't a good starting point for an mx quad. No matter what you do to it, it's still going to be heavier than anything else on the track.