Alright, here are your answers:
The 350 "Vortec" Chevy engine is the same 350 block as your TBI engine, the differences are as follows-
The Vortec engine has fast-burn Vortec Cylinder heads. These are pretty nice performance cylinder heads, and offer much better power than the old "swirl-port" TBI engine cylinder heads. The Vortec heads are cast iron heads with a 64cc heart-shaped combustion chamber, which is much better than the TBI D-shaped chamber. The sparkplug on a vortec engine sits deeper in the combustion chamber (where the heart-shape points inward) so that it gets a much better and quicker spark. This burns the fuel off faster, gives you higher rpms, and gives you more power with better fuel burning efficiency. The intake ports also have a spiral design that creates sort of a vortex of fuel/air as it enters the combustion chamber, making for great atomization; basically it is a much better "swirl-port" design than the TBI heads tried to use. These heads also made higher compression.
The Vortec engine also uses multi-port fuel injection , instead of throttle body injection. This helps a little with gas mileage, and is also a much more efficent type of fuel delivery. Instead of two injectors within the throttle body that spray fuel into the butterflies at certain intervals based on the feedback from a small handful of sensors, the Vortec engine has what is known as a "spider" injector, which is one large fuel injector that has 8 "legs" or tubes coming out of it and through the intake manifold. These tubes exit right at the intake port of the cylinder head. Using a lot more sensors than the TBI system, and the OBDII computer system, the Vortec system pulses fuel through the injector at high pressure and enters the combustion chamber WITH the air and mixes together inside the intake port.
So, with the Vortec Cylinder heads, Multi-port fuel injection, and OBDII computer, the Vortec engine is able to make 255 HP (TBI is 200 HP) and 330 ft/lbs of torque (TBI is 300 ft/lbs).
As for the mileage... well honestly, the EPA estimated fuelmileage for the TBI and Vortec engines are actually about the exact same. The people who report better mileage probably get better mileage for other reasons. They may have bettermaintained vehicles, aftermarket parts theat help get better mileage, or they simply drive their trucks more conservatively. I mean, the Vortec engine is only rated (by the EPA) for like 13.8 mpg mixed.
For matching a vortec engine's performance with TBI... Unless you really start changing some things in your engine, you are not going to be able to gain 55 HP and 30 ft/lbs. of troque to match up to the vortec engine. Hope I helped get rid of some confusion.
