I just purchased a 55 Ford F-100 via Ebay. The title shows a VIN number, but I cannot find it on the truck. Where are they normally located? I thought the left door, engine firewall, etc, but no data plate found.

 

All my research has been for a '48, but I believe Ford stayed the same at least through your year.
3 locations
1) GVW plate inside the glovebox (attached to the back of the glovebox door)
2) Serial number plate riveted to the firewall close to the passenger side hood spring. (to the right and slightly higher than the spring)
3) Serial number stamped on the frame usually the passenger side just forward of the radiator mount crossmember (in Canada and some oddballs on the driver side same area) (on the topside - visible standing peering down into the engine bay)

Legally the FRAME vin is the one that you go by, since the cab is removable and considered replaceable. Cops (VIN inspection) are too lazy to dig around under the hood and only go off the Serial Number plate on the cab. So... unfortunately a fair number of folks have cabs that have been swapped over the dozens of years and VIN mis-matches. You *should* change the cab plates to match the frame, but most people end up with cab plates matching the title and registration so leave it at that.

This is a really long thread on the 49/50 cutoff that ended up also bring in 48 and post 50's as well - lots of good info in there:

https://www.ford-trucks.com/forums/4...n-cut-off.html

 

On my 55, in addition to the glove box data plate has the VIN stamped on the passenger side top of the frame about even with the front of the door.