W2 heads on a 318

318/360/340 will all be within 22 cid of each other if you run the same crank and the bore is big enough to make similar power as 340/360.

But I don't get why you'd pick a 318 to stroke if I was going to the expense of stroking I'd start with 340/360.

If I was to build a 273/318 it would cause I'd want that size of engine.