I've seen three Dana 20's in GM trucks--two in 1969 and 1970 K5's with a straight six and a 307 V8 with three speed manuals,and one in a 1968 Chevy K-10 ,also a 3 speed straight six..all three had a weird transfer case adapter with the shifter mechanism incorperated into it..
They must be rare because I've seen at least a hundred of all the other types of transfer cases GM used ..
Huh, now it makes me wonder what is in Dad's old 70 K5 with 3 on the tree and a 250 six.