Will Chevrolet LS-1 pistons fit into the Chevy 4.3 vortec engine from a '98 S-10 How much will power output improve?

no they won't. Both the bore and the stroke are wrong. 4.3 L pistons are a 4.00 bore X a 3.480 stroke, the LS-1 engine has a 3.900bore X a 3.620 stroke. your not going to gain any power because they wont even fit the block.