[goinbig]

The cylinder walls are bored out, which requires the use of a larger diameter (bore) piston...causing a larger displacement. you can also stroke it, where the piston has an increased up/down movement (stroke) than stock.

If it's a 350, a 383 stroker is the common thing to do. You can search around for more info on them if you aren't familiar with this engine.

If it's a 305, you can increase it to a 350. A 350 has the exact same size block and everything, but it is simply a factory bored/stroked 305.

This can all get pretty spendy though if you want it done right.