Yes, you should be able to re-use the power steering pump from the 250 on the 350.
You will need to change the mounting brackets (those are completely different). And you may also have to change the pulley (I think the offset is different). The only other thing that might be an issue is the return line connection on the back of the reservoir housing (sometimes those point off in different directions depending on the application).
interesting topic. here's what happened to me. Before I got the car which was originally a 6 cyl 3 spd auto. someone installed a 350 fro an 80 to 84 c-30 truck. they must have used the truck pump on the installation. the pressure line fitting was different and it leaked no matter how tight I got it. bought a new pump w/o a pully. installed the old pully on the new pump no leak with the new pump. that's when I discovered the difference in the pressure fitting. one is an o ring seal the other is a flair seal.
Just weird Nova stuff. Maybe it will help some of us.