If you want 'plug and play', stick with a 318 or get the one you have re-built.

Is there any reason you can't pull the existing 318 and get it rebuilt? That would probably be the cheapest route and easiest. You could probably have the engine rebuilt to OEM specs for maybe $1000 or so....

As far as what engines will bolt in...any 'LA' small bock from the 60's thru the mid 80's...as long as it was carbureted. People here will tell you to jump to a 340 or 360 but that is for performance reasons. If you are looking for power, these would be better starting options. 340's tend to be harder (more desireable) to find than 360's. However, unless you get a spectacular deal on a complete engine, this will cost more than the 318 route.

I would go on ebay and search on "Mopar 318":

Here is one!

Here is another!

You can also get reman'ed engines through AutoZone or Advance auto for pretty cheap if you don't want to deal with finding another motor. If all you want is OEM performance, these will work fine.