If you want a stroker SBM the best option is to start with either a 340 or a 360 block. This allows a really good selection of quality off the shelf pistons.
Building a 318 stroker will not allow cost effective piston options. Custom pistons are really expensive and take a lot of time to get. Real pain if you hurt one.
Using a 4.00" stroke crank in either a 340 or 360 gives you a great short block, ease of parts selection, and is the most cost effective.
Tom