There was someone in toronto who swapped a 3b into an 80 - but added turbo - and apparently the work was horrible and there were lots of problems. A turbo 3B woiuld be ok in my opinion but a non turbo would be not good.
Seems like the specs on the 3B are about 117 hp and maybe 177 torque? Something like that but most agree you can add about 25% to diesel numbers to compare fairly with petrol. In any case it would probably be similar to the 3FE, which is slow but acceptable in my opinion unless you live in the mountains. I wish I had thought of the 3B when I was looking into conversions although so far I am very happy with my engine just not my transmission!
I know this doesn't really address the original question, but if you want to go relatively low power, would you not be better off swapping in a 1HZ which is already a stock option in an 80 series and should be much easier, and presumably less expensive to do? Even the 80 with a stock 1HDT was a bit slow, so I'm pretty sure that even though the OP said speed wasn't a factor, the little engine he's thinking of putting in there would be so slow it would be a factor. Just thinking out loud....
a 3B equipted 80 would be a slug, s***ty on fuel and the parts are getting harder and harder to find so bad idea
it is your time and money and if you want to do it, go for it.
you could sell the 3B drivetrain to the states for decent money and buy more fuel for the gasser
ANYTHING can be done with time and money so yah, it could be accomplished, a turbo and intercooler would help in the power department (as Louis said), the ride would be nice and the rattle would be very different in a usually quiet series of truck.