Nvrdone nailed it spot on. If you remove that crossmember the frame needs to be reinfoced = boxed or there needs to be sufficiant support to keep the weight of the cab from pushing the cab mount brackets down and letting the cab settle until it finally lays on the frame rails.
This is the most common cause of "I have done everything that I can think of and can't get my hood adjusted right" syndrome on AD trucks. If you stand back and look the cab has sagged at the front and the lines of the cab are no longer parallel with the lines of the running boards. Get under the truck and there is no clearance between the frame and the cab at the spot you measure at.
This happened to me when I went V8 automatic and cut the crossmember out even with the truck subframed. You can see it on older builds at a lot of gatherings and rod trots where you see a batch of these trucks. Cut the crossmember out without reinforcing the frame and eventually the cab will sag if you drive the truck.
The simple fix when you are setting the truck up is to make and install a pad/block right above the frame rail inboard of the original mount to carry most of the weight. Use the regular cab mount to hold the cab in place and use the pad to carry the weight.
I've got the measurements out of the assembly manual and the hood adustment instructions out of the service manual here.
https://photobucket.com/u/mr48chev/a...c-87a31c3a5b15