Were you aware that Soloy Conversions already 'proofed' the prototype of a Turboprop Skymaster?
"Developed and built under contract a prototype 600 hp pusher
turboprop installation for the Cessna 337 using an Allison 250-
C30 engine and a Soloy Turbine Pac gearbox."
Here's the URL:
http://www.soloy.com/Resources/misctext/leader30.pdf
Conroy also created a single engine turboprop conversion of Cessna 337 Super Skymaster, N1414G called the Stolifter. The rear engine was deleted and the forward engine was replaced with a 575-shaft horsepower Garrett AiResearch TPE 331-25A turboprop. The fuselage was extended to nearly double the volume available for cargo. The short take-off characteristics of the Stolifter were improved by the incorporation of a Robertson Aircraft Corporation high-lift system. The attached picture was taken on June 12, 1974. Kind of an UGLY-DUCKLING if you ask me.
If you think 100LL AvGas is expensive for your airplane... just think what all that turbine Jet-A would cost!! The turbines are just not what you'd call fuel-efficient.
SkyKing