While the LT5 is electronically fuel injected, it's still a normally aspirated, gasoline burning, spark ignition engine.
Remember in ancient times what happened when you bolted an 850 holley on a stock 350, well...the same problem can exist with a stock or near stock LT5 and a TB with big bores.
To date, my information has been that the 63mm conversion is of little or no performance value on stock or near stock 350s which see the typical street high-performance duty cycle. Additionally, on a stock or mild 350, performance and throttle response from idle up to the mid range might be slightly degraded with the net sum of slight losses at low-speed with little or no improvement up high being a "wash". I'd talk the 63mm bores over with Marc and go with his recomendations.
On the other hand, if you've got a 350 with a lot of mods and which runs safely to 7200 or so and is going to be race more than driven on the street, then, well....the 63 mm secondary bores might be worth the money.
Now, once you start increasing displacement...368, 385 and so forth, then the bigger bores become increasingly more valuable.
With a stock or near stock 350 for street high-performance the big pay-off from "top-end" jobs come in the head porting, (if it's a 2-bolt motor) the housing work, freeing up the exhaust (if you live in a state with no exhaust emissions test, include cat removal) and changing the calibration. From their the gains get progressively smaller with reductions in restriction upstream of the injector housings (plenum, air intake system, fitler, etc) being worthwhile but not getting you as much power. In addition, optimizing cam timing can add a little performance but, the idea that there's a "silver bullet" cam timing configuration on a 350 with stock cam profiles that's going to make a ton more power is urban myth.
My guess is that if you port the heads, housings and plenum, free-up the exhaust, free-up the air filtering, set the O.E. cams accurately and go with one of Marc Haibeck's cals you'll be at 400-410 SAE-corrected at the wheels. Add a set of headers and remove the cats and you'll be at 425-435. If you need cats, the right aftermarket units seem to cause little loss.
Good luck.
