The c6 has a detent (passing gear). It does not have to be adjusted perfectly or even hooked up. If it is adjusted to tight it won't shift into 3rd. To loose and it won't kick down. If you put in a c6 you'll want the roller kit installed. If you can build the trans yourself you can get the parts from an E4OD. Even if the E4OD is bad the rollers will probably be ok, just clean 'em good. If you have the trans built at a shop it may be difficult to get this done unless it is a perfomance shop.
In '89 the c6s got the rollers from the factory. It is hard to find a small block bolt pattern, '89 and newer c6 though.
The AOD housing is about 1/4" longer but, the adapters are different and I don't have them here to measure.
