An input speed sensor is required for my control systems. All Chevy 5 speed Allisons had it, but only the ’06 6-speed Chevy Allison had it. Some rare versions used in ’07 had the casting for it but were not drilled. All commercial SAE 3, round bellhousings also have it that I have seen, and there are adapters available that use it. These SAE 3 bellhousings require a different starter than the ’94-’02 Dodge Cummins used, the first gen starter may work but since they are bigger there may be clearance problems. Also watch out for the bigger SAE 2 bellhousings – they are less common but can easily be misidentified.
There are pros and cons of either the Chevy or SAE bellhousings. There are many companies making adapters for the Chevy style these days and they require less parts. The SAE puts the cooler lines on the bottom of the transmission which is easier to keep away from exhaust, and allows for the option of keeping the starter on the driver side of the engine. Both are comparable in size as far as length and diameter, so transmission tunnel or exhaust clearance isn’t really much of a factor. Depending on what adapter is used for the SAE 3, it can actually make the trans a tad bit shorter.
See the threads in this forum for adapter options for more information.