I can't say how they will fit on the previous generation Chevy and the fact that it is a cutaway, but on the later 190s and 210s 265-75-16 tires fit fine. The are taller by only .5" so my guess is that you won't even notice any difference in driving. I am super aware of such stuff and I didn't notice it. Speedo will go off 2-3mph but it isn't enough to mess up the computer in any way that I have seen.
The wider, higher capacity, tires are, IMO, a good improvement. A bit better handling, probably run a bit cooler, more load safety factor, so all good as long as they fit.
Be aware that the OEM steel wheels are not wide enough to go upsize so you would need new wheels if you have those OEM wheels and the generation of van you have used the same as the later ones.