the differences between the 1989 and the 1979 van almost makes it unfeasible to try to make the ac fit.
if it were mine, i would look at one of those universal, under dash kits that are everywhere.
i realize the dog box will be in the road, so some creative duct work would be needed, unless you get a direct fit kit.
without having the vans in front of me, or pictures of the dash setups and many other things, i can't really offer much advice on the ac swap.
if the 89 van transmission is the hydraulic controlled A518, i would seriously consider swapping that transmission into your 79 van. that shouldn't be too difficult a job to do.