A ground up restoration of a Chieftain coul;d be a very expensive exercise. At at the end of it all, you still have an unreliable pair of engines running on fuel which is increasingly hard to find in some areas.
A turbine engine overhaul may cost more, but a PT6 should go full life unless mistreated or you are plain unlucky. Garretts are also OK in the right hands. The same can't be said for blown piston engines of any make.
F406 is the way to go.