Posting this as an FYI for others.....
I bought a replacement W-1 carb for my '46. When I installed it I used a new carb insulator. The new insulator was a hard black plastic while my old insulator was some type of composite. The black plastic was too tight on the manifold so I filed it wider to fit snugly. Put on the new carb and could barely keep the engine running - needed to choke and run at high rpms. Over a period of days kept fiddling with it but it would not run properly. I figured I had a bad rebuilt carb, swapped the old one back and ran a bit better but still would not idle, it needed to be choked and would only run at a fast rpm - otherwise it would die.
After several days of wondering what I did to screw things up, more adjusting the carb, etc. I finally remembered the insulator. I swapped that back and the engine runs fine. I never would have believed that that dumb bit of plastic would make such a HUGE difference in how the truck ran, it was night and day.