Built motor $3k here by people who race turbo Miatas: http://trackspeedengineering.com/sto...63382adb638bcd

And you might have trouble hitting 300whp on a log manifold. Check out ABSURDflow or Artech for better options.

And your '99 Torsen is plenty strong.

We need more info! what are you going to use the car for(big one<--), what kind of person are you? DIYourselfer? do you have $ to spend(it sounds like you do)?
I have no personal experience with a miata pushing that much hp, besides what I have read on here. From what I have read you can boost a stock 1.8 with a real good tune safely to 250hp. The rest of the car should be able to handle that as others have said^. For around 100 more it'll cost a lot more but if its what you want then do everything right the first time. IE tubular SS maifold, 3"id downpipe (like he said^), correct sized inj that have been recently flow tested, good stand alone(megasquirt/hydra/aem), youll need a 6 speed because thats what everyone who has done this has ended up with, 1.8 rear end(rebuild to be safe), fatty clutch, meth injection? Theres little things too that shouldn't be forgotten(motor/diff mounts, cam choice, dyno time, ect.) Expect to break alot of s%^& anyways at 350hp, they always do.