My 81 B250 van with the 318 engine and 727 Trans is a bit earlier than yours so the part numbers might not match, but I'll chime in in case it helps.
I had problems finding correct parts too; the local auto shops were happy to sell me Y-pipe replacements, but they didn't fit correctly, running too close to the transmission pan. They were functional, but I couldn't use the stock support bracket, and the transmission would cook...
It didn't help that the van came with no catalytic converter; someone had custom bent the rear part. I wanted to put the cat back in just in case I ever had to get an emissions test.
After a bunch of determined research, I eventually figured out that the majority of local parts stores sell Merit brand exhaust, who simply don't make a direct replacement, and what the parts stores were substituting, the Merit #95949, didn't fit!
I finally tracked down a Walker exhaust dealer, and the Walker #40347 Y-pipe fit perfectly.
Check out the difference:
I bought the catalytic converter on eBay.
Please try to locate a Walker dealer, and your success rate is likely to improve greatly!
It may be worth checking Rock Auto to see if they have the exhaust manifolds. Hopefully it's not the passenger side one. It's a PITA dealing with the heat riser! I found a source for the parts, but it requires some precision welding.