Only reason to destroke an engine is to help make it fit a cubic inch limited class, which sometimes has a cubic inch-to-pound rule. 340 has the advantage of OEM forged cranks and the main journals are smaller which provide less friction. Otherwise, everything else is basically the same. Standard bore 4.00" 360's can be bored to the standard bore 4.04" 340 size. All else being equal, more cubic inches will result in more power.
Mopar did it in the Trans Am series with the 340 because the rules had a 305 cubic inch limit. The weight to cubic inch rules typically are found in Modified and Competition Eliminator type drag racing classes. A small cubic inch engine can be made to put out more HP per cubic inch, but more cubic inches tend to make more overall HP. Also, the smaller you go, the more rpm you need, cost goes up and reliability goes down. It's mainly a special application mod.
Someone use to, or may still do, make bearing spacers to fit a 318/340 crank in a 360 block. The main reason why people stroke a 340 is to get more cubes with what the have available. The do the same with 360's and big blocks.
This may turn into a 340 vs 360 thread, like several in the past. But that'd not the point.
