....Turbo 4.8L LSx / 4L80E / 2.75:1 9" Ford axle / 28" rear tires....
Welcome to having a build underway. I like the engine choice. What are you going to do for a crank when you start making the power needed to run over 250? The head choices for that engine are good and I'll be interested in seeing how it works out for you. Might be something I look at later also.
I'm wonder why the 4L80E? You would be stuck with one set of gears basically and not really the best ratios for what you are wanting to do. Maybe consider something like the G-Force 101A where you can change gear ratios cheaply down the road to match what you are doing at the time. Not a whole lot more than a built 4L80E and the controller for it.
You might also take a look at this thread starting on page 7 where the discussion turned to a Ford rear-end combination where you end up with 2.26 gears ....
http://www.landracing.com/forum/index.php/topic,13504.90.html
Hooley runs the 2.47 Ford 9 inch in his Stude but to get to 300 that presents some problems gear wise unless you are going to really turn the 4.8 and you don't really need to do that with the turbo.
Just a couple things to think about and good luck on the build,
Sum
I can afford to do it safe, but I'm not trying to order the best of everything, bolt it all together, and call it done. The 4.8L is not only because Holdener did 1203 HP in HotRod magazine, though the aftermath, or follow-up that never got published, was that his 4.8 continued to survive a great many more pulls over 1200 HP, still on a stock short-block assembly. I don't foresee ever getting many chances to open it up that much, mostly drive on an 800-horse tune, more likely.
I'm convinced the Ford 9.75" rear axle assembly, with 3.08:1 gears, that I intend to use for everything else but the salt, isn't necessary when there's no real traction. The 9" is easier to install, and stronger, than a Ford 8.8" or a GM 8.5", both of which offer close-enough 2.73:1, but the 8.8 is by far the most common in the salvage yards.
There are guys who've run stock 4.8s to 7500 RPM for years, even without ARP rod bolts and Comp Pro Magnum HR lifters, so the RPM can easily go 7500 with those upgrades installed. I'm mostly concerned about valvesprings, as the springs used by the 7500-RPM non-turbo guys are the same ones used for 1203 HP @ 6500 RPM. Titanium retainers will help, but I'm not doing Ti valves. The best normal exhaust valves are available for $ 250 / set.
I surely don't need to touch the heads to run 255, but probably to run 301.
I had to pass on a set of 2.50:1 9" gears because the old man refused to get himself a PayPal account. There's no other way to guarantee that nobody gets cheated unless it's live, face-to-face, in person. He couldn't understand that. On the Ford460 forum they claim that 2.50:1 was for the 460, and 2.47:1 was for the 351M, and the 400M in station wagons, or some such.
2.73:1 in the rear, 0.75:1 in the trans, and 28.6" (Y)-rated road tires puts my RPM at 7249 to go 301 MPH. which brings me to the 4L80E
I can rebuild one myself, I can get one in the salvage yard for $ 125, and they can be built to hold 1400 HP. The alternative is to spend 16 times as much for a Tranzilla from Rockland Standard Gear, and that is still 0.75:1 in sixth, not 0.68:1 like last year's ZR1, which they take first through third gears from anyway. Controlling the 4L80E is no cost at all. I already got the #411 PCM and harness with the 4.8L, and the 4.8L was offered with the 4L80E in the Express vans. I e-mailed my tune guy, Ryan Gick at GMTuners.com, and unlocking the anti-theft, calibrating the speedometer, and replacing the 4L60E map with the 4L80E map all adds up to $ 85, not bad at all. He always gets it right on the first try.
I hope I've answered your questions, and thanks for being friendly!