I'm not up on volvo's but iirc it's a unit body.. and if the drive shaft is rubing at the rear. nothing from the engine swap caused that..
now springs settle over the years from age and from holding up the weight for 33 years..
might try to find the factory ride height.. and see where your car is compaired to what it was new, it might have settled an inch or more..
you could also have a cracked(broken) coil on that side..
you might be able to install new 1980 volvo wagon springs to give little less drop when you have a full crew in the car..
I'll bet you'll find the 33 year old springs have saged and weakened over time.
install new wagon springs(if they fit) or new replacements for your model and the problem will go away..