Repower a 43' Planing hull houseboat with diesel sterndrive.

Seems good advice to me re: engines. I'd look at a Zeus drive instead of a traditional sterndrive leg. Depending on space, which could be generous on a houseboat, you may be able to engineer one that can be raised out of the water when not in use.

 

I'm afraid the short answer is NO, the reason, as mentioned above is inability of gears to handle the torque.
The Cummins 6b is a great motor, but makes its power at way lower revs than a comparable yanmar or volvo, and is a notorious destroyer of stern drives.
Volvo is making new sterndrives that are said to be capable of diesel torque, however I wouldnt jump on the bandwagon just yet, but let the dust settle until it becomes a proven product.
Depending on how much use the boat will actually get, gassers may be much cheaper in the long run anyway, you just have to pencil out the alternatives.