If you are good with welding you may want to consider making your own 'turbine' case to replace the pump case and use only the impeller, shaft, seals and bearings from a split case pump. This will allow you to design the turbine case with a much larger inlet to eliminate the restrictive pump outlet. Note that pump outlet = turbine inlet. This will get you a little more efficiency in converting the energy in the water ( head X flow ) to mechanical shaft energy.
You could even buy just the parts you need. Example shown here. This is a project I'm considering, at least on paper. Another approach might be to modify a split case pump housing with a second outlet. (inlet with respect to turbine operation) Since these housings are mostly made of cast steel it would involve welding of cast steel or cast iron. I'm not sure it would be worth it for maybe a 5% boost in efficiency. But if you like to tweak and optimize ...
