I don't know about "modifications" to the cruise-o-matics BUT I do know for certain that the C6 can be replaced with an AOD tranny by using an adapter plate. The kit comes with starter, flywheel, flywheel spacer, all bolts and the adapter plate. all you have to do is cut the "inner" ear off that is on the way for the starter gear to engage. PATC transmissions sell full modified C4's with overdrive also.
look up PATC transmissions for a C4 with overdrive.
If you want to swap a c6 for a AOD, here's the link :
Ford FE Transmission Adapters
these guys sell full trannies also, pricey I think...
FB Performance Racing Transmissions - Chevrolet, Chevy, Buick, Olds
lentech does also, but no prices posted...I guess they price whatever whenever...
good luck.