Check this......
Mike

 

Looks like someone swapped a 78 cab onto a highboy chassis. I'm curious to see what the VIN on the frame says. Also, measure the distance between the frame rails. Highboys are 34" and the regular f250's are 37". But after looking at your pic, I'd be money that someone swapped cabs at some point.

 

Is that measurement at both ends? Or just the rear has the narrower spacing?

Paul

 

33 1/2" width between the inner frame rails behind the cab: 1961/72 F100/350; 1973/77 F250 4WD (High Boys); 1973/79 F350 Regular Cabs on the 140" wheelbase & Cab & Chassis.

37 1/2" width between the inner frame rails behind the cab: 1973/79 F100 2WD/4WD & F250 2WD; Some 1973/79 F350's except as noted above.

1975/79 F150 2WD/4WD; 1977 F250 4WD from serial number Y20,001 (not a High Boy); 1978/79 F250 4WD; 1979 F350 4WD

 

Your cab vin starts with F15. Meaning it's from a 2wd F150.

 

There is no way = NONE that a 1978 is a High Boy. The last High Boy was assembled February 1977.

 

That was mikes cab, not the OP

 

I see now. My mistake.