340 the hard way; Stroked 318

When you had this 318 apart did you happen to measure the distance the slugs are down in the hole? you may be able to use the ones you have.
The last 318 I measured they were .110 in the hole, so the thought was to use a 360 crank - take .025 off the top of the 318 pistons and end up with a 10.0CR motor with open chamber heads.
As you have closed chamber heads you may have a CR problem -too high.

IMO you need to do some carefull measuring of components to get some data on what you have before you purchase pistons etc.
block deck height? (not all small blocks ended up 9.599 - I had one that was stock 9.582)
top of piston to top of deck?
cumbustion chamber CC?
Altitude at your location? I'm at 3200ft so a calculated 10.4CR still runs fine on pump gas.
Idealy you wouls want to end up with a zero deck motor so a ,040 gasket gives you the proper quench, The same can be obtained for example with a piston sticking .014" above deck and a .054"th head gasket. Food for thought.