Mopar 340 Small Block

Without question, the 340 Mopar was Chrysler's premier performance small block. Unfortunately, it was produced in limited numbers, and a good rebuildable core is very difficult to find. Most people these days wanting to build a high performance small block Mopar go for a 360. The Mopar 360 is plentiful, and with modern aftermarket parts can be built into a very decent performance piece. However, I believe the smaller main journal size of the 340 Mopar block is a big advantage over the 360 if you are building an all out performance or racing engine (if you can find an original or don't mind paying big bucks for a NEW retro 340 block).

When the 340 Mopar was introduced, it shared the short stroke (3.31") of the 273 and 318, along with the journal sizes of those engines (2.4997" main, 2.125" rod). Combined with a bore of 4.04" and a set of free flowing heads, the resulting combination was an engine that could rev freely. The 360, by contrast, had a smaller bore than the 340 (4.00"), a longer stroke (3.58"), and worst of all the main journal size was increased to 2.81". Increasing the journal diameter results in higher bearing speeds for a given RPM, increased heat, and may have been one of the factors contributing to the 360's reputation for spinning bearings (at least in stock form). Although the 360 can be built into a good performance engine, especially with modern parts and building techniques, the 340 block is a better foundation to build on - if you can find one.
There are 2 reasons that 340 blocks are so hard to find. First of all, they were only produced for 6 years during a time when most people preferred a 318 for economy or a big block for performance. In a sense, you could say they were ahead of their time. Second, because of their reputation, they tended to get used and abused pretty hard, and therefore worn out. If you can find a good rebuildable 340 block today at a reasonable price, count yourself lucky.
If you can't find one, you don't need to worry too much, as the Mopar 360 will serve you well as a pretty potent street engine. If you want to go racing or you're restoring an original 340 car that "has" to have the original displacement you can get new blocks from Indy Cylinder Heads (317-862-3724; I'm not posting a link to their web site because Google reports it as hosting malicious software). They're pricey at around $2500, but they're capable of producing much more power than any "factory" small block Mopar.