Stroking a 318 engine

Stroking a 318 is the same as stroking a 340 except the 318 has slightly smaller bore. Other than that they are the same.

If you are talking about a 4" crank then stock length rods are used with shorter pistons. If you use stock rods you will have to notch the block. If you go with Scat I-beam rods they are smaller and no block notching is needed. A 4" crank with Scat I-beams and the correct pistons will just drop right in with no block mods needed.