The differences in the Convair propliners was in passenger capacity first and foremost, with the CV240 being the smallest. Avionics, range and passenger capacity were upgraded or increased with subsequent models.
It is my understanding the CV240, 340 and 440 were piston powered, and the 540 was as well. The CV580 resulted when the original PW radial engines were replaced with Allison turboprops, like those found on the Lockheed L188 Electra. There was also a CV600 which was powered by Rolls-Royce Dart turboprops. Freighter versions of the CV580 HAVE been stretched to produce the CV5800.
As far as re-engineing with PW turboprops from a SAAB 340, it's possible (hell, you could probably put jet engines on a CV5800...
) but not likely, given the age of the aircraft.