318 to 340??

The 273 thru 360 are basically the same block externally, but have different bores and stroke to come up with the different displacements. The heads, manifolds valve gear, and torque converters vary. Most parts interchange, but some don't without without the right parts. Like the 360 Magnum heads use a slightly different intake bolt arrangement I believe. So go for it! Do some research before swapping heads, manifolds, converters etc.