318 to 360- what is interchangeable, and what do I look for in a street motor?

Ok, here goes.

The transmission will bolt up, they have the same bolt pattern since they're both small blocks. BUT - 360's are all externally balanced. Which means you will need a 360 damper, and an externally balanced flex plate or torque converter if its an automatic, or a externally balanced flywheel for a stick.

The heads, intake, and carb will physically fit onto the engine. However, the intake and exhaust ports and valves are smaller for the 318's. The valves are stock at 1.78/1.50". Which will mean a very asthmatic 360. The intake also has smaller ports, and you guessed it, the carb is smaller too. The 318 combustion chambers might also be smaller depending on what year your 318 is, but any boost in compression will be totally overrun by the lack of flow. The only exception to this would be if your 318 was one of the police package deals that came factory with 360 heads and a 4 barrel manifold. I'm guessing its not though.

You would also need a new engine mount, the passenger side mount is different for a 360. If your 360 doesn't come out of a car, you'll also need a new oil pan, since the 318 had a different size rear seal.

I believe the rest of the accessories are the same. But I may have forgotten something. I haven't personally done the swap, but I've read about it enough times to remember what I've got so far.

On that note, this has been covered on here at length more than once, search something like "360 swap" and I'm sure you'll get tons of reading material...

A 340 swap is a little different, but it depends on the year. Forged crank 340's were internally balanced like 318's, but they only came that way from 68-71. So, if you got a 68-71 340, you could skip all the external balance stuff. BUT - early 340's had high compression pistons that protrude above the deck, and a compression spec'ed at 10.2:1 with a 65cc chamber. If you have small chamber 318 heads (60cc heads for example), you will end up with a compression ratio that probably won't run well on pump gas and a possibility of piston/valve clearance problems. Depends on the year of the 318 heads. And you'll still have tiny valves and ports, so it won't breathe either.

'72-'73 340's had cast cranks, were externally balanced, and had low compression pistons. So, a cast crank 340 will basically have all the same problems as a 360 swap except for the oil pan (318/340's use the same pan) and the passenger side engine mount. The low compression pistons solve the compression ratio problem with the 318 heads, but the valves and ports are still asthmatic.

Search 318 vs 340 for more info on that one...

Either way, the cheapest thing for you to do would be to find another 318 for now. Or find a complete 360 long block, so you don't have to mess with any 318 parts. You'd just need the external balance items, the oil pan and engine mount.