Can't give you the correct numbers off the top of my head, something about old age.....

Anyway, the frame width of the Dakota from the kickout behind the front wheels to the rear axle is straight and fits just inside the original frame on the 39 plymouth at the firewall position. The Dak frame is also flat except for a raised bolt on crossmember that the front of the box mounts to (and the fuel tank mounts to) until the rear axle kickup. I have a 90 Dakota sitting outside I can get a decent measurement from tomorrow (if I remember). The Dakota frame hight also remains the same size and fits just inside the rail hight of the 39 Plymouth. (i'll measure that as well). The length of the Dakota frame from the front kick out to the part of the rear frame that kicks up to mount the box on depends on whick wheel base Dakota frame you have. That measurement may be a bit harder as my 90 is still a functioning truck. The one I mounted looked like it was made for that frame swap. I will also get the track width of my truck. Gene