Ok, here's the scoop on the LA smallblock. 318 has a bore of 3.91" and a stroke of 3.31". The 340's stroke is the same as the 318, but the bore is larger-4.04", and no you can't bore a 318 out to a 340. Not enough iron in the block. The 360 is kind of by itself. Bore is 4.00" and stroke is 3.58", plus it is externally balanced and has a larger main bearing size than the 318/340, so cranks wont swap between the three. A 340 crank will fit a 318 and visa-versa, but the balance will be off even though both ar internally balance (except for the 73 only cast crank 340....following yet?).
The 318 heads have small ports and valves, they will fit a 340 or 360, but I wouldn't recommend it. the ports are too small for good breathing at high rpm on both of the larger motors. 318 valve sizes are 1.78" I, 1.50" E. 68-71 340's used 2.02"I and 1.60"E, and 72-73 340 and all 360's used 1.88"I and 1.60" E valves. There have been some conflicting reports of what heads were fitted to the 318 4bbl police package back in the 70's. Some sources say they use the standard 318 heads with the 360 4bbl intake manifold, others say that it used the 360 heads too. Either way both the intake and the heads will swap onto a 318.
The left side motormount is different between the 318 and the 340/360. The 340 and 360 use the same mounts, while the 318 is all by itself (unless you have a 273 then that would be the same). The 360 oil pan is different from the 318 and 340, it has a smaller rear sealing face. Depending on the year, the 318 had a light duty con rod or a HD rod that was the same as the 360. The 340 rod is similar to the 360, but is setup for floating pins.
And these are just a few of the basics, but pretty much everything else is interchangable. I'm assuming the original question was asked because you want to replace a 318 with either a 340 or 360? Thats an easy swap, but if you have a 318, why not do some low-buck performance upgrades to wake that 318 up?
Mopar Action did an article a couple of months back where they took a stock reman 318 from the local Pep-Boys and added a mild cam, headers, a dual plane 4bbl intake and a 650 Demon carb. Pretty much left the heads and compression alone, and guess what? The got 283hp out of that 318 that dyno'd stock at 182hp. Not bad huh? If you think about it that was about 8hp more than the 340 was rated for in its heyday (yeah we all know mopar under-rated it but hey it looks good on paper).
Whatever direction you go, good luck. The mopar smallblock is a great motor, good hp and reliability. Just like our /6 too.
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