Hey Marty,
I recall reading of a Ford powered Cruisair some years back in the newsletter. I do not recall, nor do I know, what you have to do to get a certificated aircraft moved into the Experimental category. All I know is that there is more to it than simply proclaiming this change to the feds.
Previously approved changes have most commonly involved engines as you can imagine. A long time back many owners stuck O-435 Lycomings in their Cruisairs, but that's not a great idea today. Franklin 180s have gotten past the federales, and I know of one case where a Franlin 220 was approved. Far and away, however, the Lycoming O-360 has been the most popular and probably the most intelligent choice for those who don't want an orphaned engine up front. Usually this involves employing a different cowling: Cherokee, Commanche...I forget which one.
Lots of people grew tired of those 36-37 hand cranks to get the gear up or down and have gone over to hydraulic hand pumps or electrical systems.
The prop is a problem as good Aeromatics have grown scarce and Tarver's shop is not exactly an off-the-shelf facility. Thus many Cruisair owners have opted for fixed pitched props.
The fundamental problem is that we are not a type club with thousands of aircraft flying, such as the Luscombe, Cessna tailwheel, Champ, Cub, Taylorcraft and others are. Thus there is no comprehensive collection of 337s and one-time STCs available.
As for the TC, last I heard, G.M.'s son, August Bellanca, holds it for Cruisairs. The TC for Cruisemasters is held by the current incarnation of Bellanca.
In short, I'm not sure that going the Experimental route will help you, nor would it be easy, alas.
Jonathan