Most of that is old wives tails. Most of the LA engines will take a .060” overbore, 360’s included. I’ve heard of 318’s going .090” over. Like everything else made back then there were tolerances so on any LA block it’s good practice to have a sonic check done if you’re going much past .030”, core shift happens and there are blocks that won’t go .060”. The ‘68 340 in my Duster is .060” over and works great.
As for the 5.9’s, I wouldn’t be surprised if they were a little more “economic” with their wall thickness. But probably more consistent too. I have heard the .020” over limit before, but I don’t have one so I never bothered to investigate further.
The 340 blocks do get a little more money, just a nostalgia thing mostly. I’ve seen them go for a few hundred bucks all the way up to a thousand. Depends on condition and the current bore diameter. I assume some of the earlier ones go for more because there are probably folks out there looking for date code or even matching number blocks for some of the more valuable 340 cars.
Funny, I’d heard to look for early castings but not “1”. Reason being it usually took a couple modifications to the mold for them to get things sorted out. So like a “3” would be great. Obviously by the higher numbers the mold might be getting used up and those wouldn’t be great either.
