Not really sure, but, on a 59, the BelAir/Impala cars are a couple of inches longer in the body than the Wagon/Delivey/ElCamino.
Same frame is used for all of the bodys in 64 (convert is different), wheelbase is is the same for all bodies.
The length difference comes in
play when designing a tailgate that opens properly on the 59-60 cars.
Also the rear bumpers are quite a bit lower on the wagon and Elky cars.
If there is a difference in the contour, it may be just a matter of trimming the lower portion of the "behind the rear wheel", if all you ae doing is replacing the sheemetal.
Any difference is going to be behind the rear wheel---and it will be a very small difference.
Not sure if the bumper is located different on the wagons on the 61-64 cars, but if it is, trimming an Impala quarter panel and moving a couple of lines should not be a big deal to someone that works with metal (NOT ME--I have a body shop guy)