yes, in fact, that is what the DB conversion is based on - the same T56 6 speed that I have in my 97 Firebird Formula and 99 Camaro SS, capable of handling horsepower mods quitely nicely I might add. In fact, it is basically the same 6 speed in the Viper only the Viper has different gearing and tailshaft.
I have not personally tackled the T56 conversion yet, however, it appears very similar to the T5 conversion of which I have personally done several of. The last listing I got from DB was around 2600 bucks for the T56, bellhousing and shifter. Of course, you have to crank in the clutch activation mechanism, either clutch or hydraulic, as I do not think there is a bellhousing available to retain the original "push" Z-bar actuation, although I am not sure about this. Check out Darkhorse's website and see what they have to say - they are very knowledgeable in the restomod area, especially with 5 and 6 speed conversions in early mustangs....
randy