Man! I can't believe this debate is still going on. 340 vs 383 is an apples to oranges comparison. The 383 was used as a base engine for many muscle car applications. The 340, along with the 426 hemi, are the only 2 engines that Chrysler specifically designed for performance applications. That makes the 340 a very desirable engine. Collectors are catching on and in some cases, 340 cars have equaled or exceeded prices for 383 cars...or so it seems. You can get insane amounts of power out of both engines. However, you can stroke the 340 to a streetable 416" while taking the 383 to a livable 451". I sure their are more experienced engine builders on this forum that can add to this.
I, personally, like the 340 in an A-body. Theres not much room even with a small block. But if I had an E-body though, I would go big block, 383-440.