Where can I find the VIN on the frame of a 78 k20?  

Just curious, but why do you need to know?

(I don't know the answer, just wondering)  

The body of the truck is a 81 and the frame and everything else is a 78.  My insurance company say the vin on the body is wrong.  I need the vin for the frame.  

Got it.

What state are you in? I know here in CA the Highway Patrol, which does inspections when VIN verification is needed, has a database of where all the VINS are on most, if not all, vehicles. I'm sure there is an agency in your state that can help you out. Or maybe even a knowledgeable mechanic at a local dealer can help out. I'm pretty sure it is in an obscure spot like the top of the frame rail.  

You are probably not going to find a VIN on the frame. You might, however, find a frame serial number that could be anywhere from three to six digits, and maybe more. I'm not sure where you'll find it on your truck, but I seem to recall that old Ford pickups used to have a number stamped on the frame at a location right behind the steering box, and also on a couple of crossmembers (which you can't see with the body on the frame). So take a look around the steering box and/or the driver's side front shock mount. With 25 years of gunk accumulated on your frame, you may need to do a little cleaning to find it . . . and I'm not guaranteeing anything. But good luck anyway, and hope this helps.