Shawn Coyle-
"The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers"
The torsional strength, stiffness and low critical speed of a long, small diameter driveshaft passing from an aft mounted power turbine to a front end PTO presents a very difficult design challenge. Take a look at the lengths Turbomeca went through to avoid the issue. They located the PTO gear drive where it is surrounded by hot exhaust duct:
Pratt & Whitney took the same approach as
"those long gone Allison engineers" with their PT6. They put the power turbine in front:
I was working on the design of a turboshaft drivetrain a while back. The engineers at the engine company were very nervous about any torsional vibrations or dynamic loads in our gearbox because their drive shaft only had an ultimate torsional MoS of 50% at MRP.
Regards,
riff_raff