Simply extracting the control structure of the assembly via automation is a huge win. This type of assembly is incredibly dense - it was written for machines with at most, 100's of K of memory. It relies on highly non-local side effects which are very dependent on the data.
Things like a reference to a status register bit that was possibly set as a side effect of an instruction 30 or 40 instructions back, but then could have been modified by three or more other instructions, none of which are control flow instructions. All depending on knowing that certain patterns do/do not ever appear in the data.
After the control flow is extracted and made visible, then you can get to things like numerical emulation of the exact oddities of S360 code.
Once you have all those things done, which you will not ever, ever, be able to do by hand, then you can take the resulting generated Java and begin re-implementing.
The work being described has to happen before the suggestion above can even be approached.