Even slower if you use a chain to connect the gearbox to the Spit with a small chain sprocket on the gearbox, and a larger sprocket on the spit.
The spit needs to have strong bearings, and be removable for putting through the animal carcase,
I would make the sprocket on the outside of the spit bearing, so you could lift the opposite end of the spit, which would provide slack
in the chain, allowing easy removal of the chain, then lift the spit off the bearing support columns..
I helped build several BBQ rottissery [sp?] setups years ago, but don't have any photos.