I'm not so knowledgeable on this sort of engine conversion method etc, previously, I've had stuff like this done by people who do know !
However, you are wanting to go from a stock 280'ish bhp to around 500 bhp at the crank, for a 'budget' way of doing the job ? - I think you may well be asking too much here! Keep in mind Alpina did the 4.6 and 4.8 litre conversions on the BMW 4.4 engines and got somewhere around 350 and 375 bhp at the crank after doing their work - I believe they DID use Forged components etc so as to keep the potential reliability long term of the engine. Looks like you are asking/wanting another 100+ BHP and I'd guess that the amount of money you are going to spend to get that, reliably will be significantly more than if you supercharged the engine. However, you say you don't want to go the FI route, so to get around 500 BHP out of this engine you are going to (most likely) need to drop big money into getting it.
Take a look around as to what others have done and their power gains etc, don't forget your budget will also need to include any other uprated aspects to the engine, probably a stonger gearbox and diff. I'd also say you'd need to uprate your suspension and of course a BBK as well, which will all bump your pricing up substantially.
Lastly, I may well be totally wrong and the engine can be relatively easily and cost effectively taken to around 500 bhp!