First, I don't think the AOD will bolt up to the 351, so you'll need to use the tranny that came with the 351. This will require custom work on the drive shaft and the tranny crossmember.
Second, I think you will have some problems will the oil pan of the 351 fitting in the van, take a look at the two different oil pans and you'll see what I mean. They are shaped differently, probably requireing custom cutting and welding to match the top of 351 pan to bottom of 302 pan. This of course will require a special oil pickup tube. All possible but very expensive.
I ran into all this when I considered stuffing a 400M into my 85 clubwagon.