Cant imagine there are many of those engine gearbox combos still about

but a carbs tend to get the bad press when its often something else which is wrong! have you checked the ignition timing? also as mentioned if someones just lifted a whole setup from another engine of different size or compression you can get odd effects, although in general idle isnt usually the problem as thats adjusteable -its usually progression as you pull away thats worst especially if people have put the breathers and vacuum pipes on all wrong.....
Could you send some pics of the install and the van as I have a recollection of Moting a similar spec van in Birmingham thats idled horribly about 15+ years ago! in fact its idle was so far all over the place it destroyed the vacuum so the brake servo wouldn't work properly, I thing I advised its owner that it was lethal!
Ollie