1956 Chevy trans ?

[PDW HOTRODS]

I just got a 56 Chevy 3600, and I'm trying to learn a little about it before I street rod it. It's an original V8 truck, 265. The code is GC wich is for an overdrive. It seem to have a 4 speed trans, but there is another lever to the right of the shifter about the same hight as the shifter. There is also something like a drum behind the trans. What is this stuff? The engine runs, but I will be using a Camaro subframe and rear, so I won't be using the engine either, no side motor mounts anyway. I won't need any of this, but would like to know a little more anyway. Thanks.