Rear crossmember

I am building a model A frame from scratch and using original front and rear crossmembers. I am extending the frame to better support the pickup box, so the rear crossmember is not going to be fastened to the rear of the frame. I have Z'd the back of the frame from behind the cab 4" and I am running the frame rails parallel going back.
I am trying to figure out how to mount the rear crossmember. Is there any reason to leave the outside ends on it, or can I cut them off and mount the crossmember between the frame rails? I am channeling the cab but not the box.

Can you post some pics? I cant seem to visualize it in my head

Here you can see the setup I have going. You can see how the rear crossmember hangs over both sides of the frame. Right now it just clamped on because I am not sure exactly where it will be, but as is, it would have to sit above or below the frame rail. If I can cut it off, I can weld it between the frame rails. I am not sure if anything is supposed to attach to the outer ends of the crossmember. I suppose I could just weld on a tab for anything that does need to attach there. On the original, the top of the crossmember is even with the top of the frame rail.

It is fine to trim it to fit between the frame rails. That is how they are put into a '32 chassis as well. The outer tabs of the crossmember in stock form were simply body mounts, nothing more.

Thanks.
The whole rear of the frame will be the body mount for the box.
Time to dig out the cutting wheel.