If this is an LT1 engine, it won't be from a truck. It should be from a Camaro, Corvette, or possibly an Impalla SS. In general the 350/5.7L engines on trucks are not set up for 'performance' like they are in the cars and yes, the transmissions are geared differently too. If you do happen to pick up a truck motor sometime, you will most likely have to (or should have to) replace the camshaft to get the rpm range higher, suitable for more performance, replace the intake and carb, although many trucks have a 4 barrel carb already, and replace the exhaust manifolds with headers. Many trucks engines have lower compression heads, which don't flow like the performance heads in the cars. Keep in mind that all of this I am telling you is "in general."
Now, this list is not inclusive--I would also replace the timing chain with a double roller unit, the rockers with roller rockers, pushrods, etc. More performance means more money and more/different parts many times.
Again, if this is a true LT1, it may be fuel injected and computer controlled. What you end up buying changes a lot of things, so let us know what you end up with so we can help.
For buying an engine, my rule of thumb is to buy the best performance engine I can afford at the get go if I have to settle for a stock engine in a Chevrolet vehicle. You can always build up from stock as time goes on.
Davy