383 pistons?
Engine displacement is not determined by rod length. Your 350ci LT1 uses a 3.48" stroke crankshaft paired with a 4-inch piston (using 5.7" rods). To convert your engine into a 383ci, you'd need a 3.75" stroke crank paired with a 4.030" piston -- and you have the option of running either 5.7" or 6" rods (which will affect your piston choice). Naturally, running a longer stroke crankshaft and a larger diameter piston will require machine work and clearancing of the engine block.
I would suggest you pick up a book on engine building -- it's not quite as simple as throwing pistons and rods in there. You have to take into account the compression ratio, deck height in respect to the crank/rod/piston travel, how much money you're looking to spend, etc.


