Parts like forged pistons are not available for the 4.9, unless you want to have a set custom made which = $$$.
As for boost, you'd only really need to run 6-8psi to start making good power. My future plan for running a turbocharger is to just run 8psi (with a front mount) and just hope nothing goes seriously wrong. However, I wouldn't recommend this for most people because in the event that something does go horribly wrong I have at least two replacement 4.9's and three replacement trannys.
Either way, if the engine is in good condition it should be pretty tough. The weak link in the chain is the transmission though, it's going to be the first thing to receive love if I decide to run a turbo.
Yeah I was planning on 6 or 7 lbs. The only problem is the exhau$t. If I go with a cold air intake, an upgraded ignition system, headers and true duals, and a wet shot of nos(about 100 shot) most of my money will still be sent at the exhaust shop. LOL. It's looking like $$$ either way I go.
The only really bad choke points are the muffler (reverse flow sucks) and the exhaust manifolds. Unless you're trying to run at 6000rpm, the stock manifolds are fine for the amount of flow that will be coming out.
You would need to get a GM 2-bar MAP sensor (simple, plug and play part), some higher-flow injectors (injectors from the 5.0L Mustang are fully compatible) and maybe an auxiliary fuel pump. If you have some 30lb-hr injectors or so, then you could get away with not getting the aux pump, but they'd need to run at a higher duty cycle.
Some ECM tuning may also be required, but I'm not sure.
The most powerful 4.9 liter out there is running stock injectors, stock fuel pump, and stock fuel pressure regulator, it makes 300hp and about 400ft/lbs to the wheels on 7-8psi. The 4.9 is a very well built motor and will give you no problems as long as everything is put together properly, the owner of the above engine has put 400-500 1/4 mile passes on the engine, has gone through three or four transmissions and by his accounts the engine still looks unused internally. If you are going to tinker with air/fuel mixtures, I suggest the Aeromotive adjustable fuel pressure regulator and the Bosch 19lb injectors, along with wideband tuning.
30lb injectors would be extreme overkill. The 4.9 runs very rich to begin with, by adding more air you're balancing out this issue. My advice would be to concentrate more on more air and ignition, and add a narrowband air/fuel meter, which will alert you as to whether your engine is running too lean, but not much else (not good for tuning). If you find that after your mods your engine runs lean, that's when I would go to the Bosch injectors. Remember also that the Bosch injectors equate to 23 or 24lb/hr injectors because of the different measuring pressure between Ford and GM, so you need to upgrade to an adjustable FPR as well at the same time - And tune.
Nitrous is not as good of an idea as turbo. If you want to do something nobody has done, I would go supercharged. The perfect spot to put a centrifugal supercharger would be in place of the A/C compressor, and nobody would know it was there even when you popped the hood... Unless they looked hard.
Who has the turbo 4.9 that you are talking about? My '90 Eldo Biarritz just turned 82K and it is time for more HP. I have saved the money to purchase the turbo, etc. and I have been researching the size of turbo that I'll need. If he has been running the stock injectors, fuel pump, and FPR, I am very interested to know how he is doing it. I have found a mask for the '90 PCM on Tunercats and Moates has some stuff to offer also. Somebody help me please as this is not a pipe dream, it's happening this summer.