Here ya go.........
The first is the top of the frame rail to the upper two bolt holes on the radiator support
This is the measurement from a line drawn through the upper bolt hole of the radiator support to the bottom of the radiator support (on the inside of the support - add material thickness if you want it to the outside)
Between the radiator support and the frame rail, there are shims to align the front end of the truck to get the fenders and hood fitting properly. Your drawing looks like you want a dimension down to the cross member that supports the radiator support. This is a measurement from the reference holes in the support to the cross member where the lower bolt goes through it from the support
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This is the setup I used to do the measurements - you can see the closeout piece that goes on top of the radiator support. This is the part that will interfere with a Chevy radiator installed in a GMC unless the radiator is moved back so the neck clears the support.
Here's a photo of an original GMC radiator in a stock truck. The GMC engine was a little longer than the Chevy, so GMC made room by pushing the radiator into the core support. This is why there is a gooseneck filler on the GMC and not the Chevy
Hope this helps you some