Remember too, as we discussed a while back, you want to go with a too big turbo on a 4.9L.
You have a base compression of 9.5:1 with a small cam. The cyl pressure is high enough already. A properly sized turbo will do 1 of things, blow head gaskets, ping uncontrollably in boost (even unheard pinging is deadly), melt pistons due to lean mixture (someone makes a fuel pressure regulator that ups pressure 1:1 ratio with boost, I think I was saying earlier that you needed 1lb more injector per lb of boost, I meant 1 psi more fuel pressure to 1 lbs more boost, sorry! Else the boost will be pushing back the fuel pressure, so every PSI higher the boost is the fuel pressure is that many psi LOWER, remember, the fuel pressure is maintained based on atmospheric pressure, not intake pressure). The engine will start falling off in cyl pressure around 3500-4000 with that cam. With a larger turbo you want boost coming on strong by 2500 to 3000 rpm to flatten out that torque/hp curve.
Check with the supercharger companies. Magazines show them all the time too.
So calculate how much fuel the stock 18lb injectors can support and see how much you think you will get out the 4.9L boosted and make sure the 18lbers will support that much power. Plan for a 12.3 or so fuel mixture, not 14.7. Turbos need extra fuel to keep from melting pistons. Been there done that, I have a reasonably designed 301 Turbo that DID get enough fuel and it will leaned out enough that I melted #7 @ 100+ mph......
So your TBI fuel will have to be boost referenced for sure, the stock fuel not really, as the TBI will support more than enough. But TBI's need around 10 psi, the PFI needs 45. Any thoughts on managing the fuel pressure? I don't know how much pressure is in the return rail, if it is higher than the TBI's outlet return rail, it will not be able to manage it's pressure well, it at all.
My 301 Turbo had a Garrett TBO305 with a .82 A/R. Which is ok for, but there are so many other restrictions the 301 never could live up to its potential. Once I get the $$, I don't see why a 350-400 hp 301 with a carb draw through system would not be EASY. As it was with as bad as it was with 7.5 psi I was putting 290 HP to the ground. No intercooler, just a M4ME Q-Jet, special HEI and a pathetic set of heads and intake....