The house being from the 50's and ABS stubbing out of the floor doesn't jive. The piping was obviously added in after the original construction. The first step is to confirm that there is a drain trap below the floor, when you pour water into the pipe can you see standing water several inches down? You should.
Depending on the design on bottom of the pre-fab shower base will determine how much cement needs to be opened around the stub out, which is not that big of a deal.