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When I was young I did not mind having to screw around with carbs and ignition in the middle of the dark and rain. Now that I am older it is nice to just go out jump in and drive.
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What he said. I put EFI on my truck in the early '90s. I've made changes to the system over the years because I wanted to do it but the amount of adjusting and repair work has been minimal compared to carby systems.
300 - 350 hp makes a truck that can play with others without getting into too much trouble.
I think you're making a bunch of expense and work for yourself with the combination you've chosen. The THM 350 and 400 don't natively attach to the 5.3. You'll need an adapter to make that happen. And the 350 needs a mechanical connection to the throttle body so you may be playing games with cable length and brackets. Then there's the $3k to get the OD unit which would buy you a transmission with OD already incorporated plus a few cases of beer as well. Most of the LS style carbureted intakes are designed for racing which does not involve lots of time doing stop and go driving or part throttle operation so they don't have features designed to help heat and vaporize fuel at low speed and lower engine / ambient temps. Real street carby intakes are designed to prevent icing and aid in vaporization. And the aftermarket replacements aren't cheap. Finally, buying a computerized engine planning to throw out the computer, EFI, and trans controls only to buy an aftermarket computer and TBI, well, that seems like throwing out the baby with the bathwater. Why bother with the 5.3 at all?? The 5.3 isn't an all out power engine but it's a cheap engine that makes good power in factory trim. Undoing everything that makes it a good swap engine only to replace the programmed and working system with one you need to tune and adjust seems like a waste.
There's nothing wrong with using OE electronics. Avoid aftermarket computers.
4L80E is a very strong transmission with good long term durability. They'll see 300k miles in our vans without needing a rebuild primarily doing stop and go driving. Trans builders can make these transmissions into full manual shift units if you really want no electronics.
Finally, stock EFI manifold for 5.3 isn't the only choice for EFI.
Holley has a modular system that allows you to combine runners and plenums.
http://www.holley.com/types/LS Modular "Hi-Ram" EFI.asp
Edelbrock's Pro-Flo XT manifolds don't look anywhere near stock.
Just some thoughts.