Large rpm drop

Have you set the T port synch on the carburetor? If the throttle plates are into the slots over square, that could do it.
At one point it was all set. Wouldn't idle in gear so I started messing with it and couldn't tell you what it's at now. I'll check when I get a chance. I was almost at the point of drilling holes in the throttle blades then decided to try a converter. Then realized I had a lockup. Then realized maybe it's trying to apply lockup all the time since I hadn't felt it ever lockup and I have no idea about the valvebody other than it's got a kit in it.