Where did this trans come from ?!?

[SeanPaintsFlames]

I picked up this 66 fleetside swb last week with a 350/4 speed in it. I'm guessing that the truck was originally a straight six, and still has the factory rearend in it, because the 350 screams in 4th gear going 55. I'm looking for a new shifter for it, but I don't know what it came from. Somebody told me maybe a Nova or Chevelle since it comes through the floor sort of diagonally. Anyway - everything I've come across has reverse top left, but this one is bottom left. The trans has "L120" cast into the side of it. Whatcha think? I'm just a painter, so I'm constanly having to learn engine stuff!