.040" should be fine, but I always sonic-check a block these days.
I had a standard-bore 318 that wouldn't have safely withstood .020" halfway down #2 in the 9 o'clock position. Otherwise, it would've gone .060" easily. A few years ago, one of the Engine Masters Vintage class entries was a Poly 318 that used an LA block (he couldn't find a proper Poly) and the builder claimed .090" was rarely an issue on 318s through '76 or so. Would I do it? No. I agree with the others that have stated keep the walls as thick and rigid as possible. A couple of cubes will not make up for wall flex. The 340 for my Challenger is only .020" over because it was clean there.
If you're hanging around the kind of people that refer to their 350 and 400 Chevrolets as "355" and "406" just because it's been bored .030", find smarter friends (or more confident in their manhood). I've never heard a Mopar guy say he's got a "four-forty six" unless it was immediately followed by "pack". :p A .040"-over 360 is still a 360 to me.
