My experience with these manifolds is that seem to fail thru overheating or thermal cycling more frequently than others. Late 70's small block fords had similar problems.The cost of weld repair and/or resurfacing is hardly worth it. You might just consider a good set of headers and solve the problem? regards, chip.
I agree with Chip...try and find a set of headers for the engine.
Here in Toledo Ohio there's about 20 or so junk yards. I to have had some problems finding parts for a similar truck with a 318 but have never been stranded. You can always substitue a similar part.
Your local library will have yellow page phone directories of most major cities in the U.S. Try calling up some junk yards in other towns and see if you can have the part mailed to you.
Lastly...your right, there were many 318 engines produced between 1980 and 1985. Although Dodge Rams didn't catch on in the early to mid 80's like the Ford's and Chevy's did, you still shouldn't ever be stranded finding parts for that engine.