Here's a quick diagram I just whipped up in Photoshop to (hopefully) illustrate where to measure. However, the measurement actually SHOULD be taken where the orange line is...which is where it would be welded up to the truck frame:
The full-size cars have a 'perimeter frame', in that it goes around the outside perimeter of the body, instead of just straight back under the cab like ours does. It's the front frame section that you'd want to be the same width as the truck frame. You'll cut that front section off where it's marked in orange and weld it up to the truck frame there.
Here's a shot I took of the '69 Galaxie front end and then Photoshopped to illustrate that same point:
In this picture, it would be cut off at the red line. The red arrow points to the short stub used to hold the transmission crossmember, which would also need to be trimmed off.