I've made my own crossmembers by getting a big channel folded up and then fabricating everything to go onto it. This included four long "tabs" that were welded into the channel - or crossmember - and they were used for attaching the crossmember onto the sides of the chassis rails. They were really extension plates that overlapped the chassis. It was easiest to weld them into the inside of the new crossmember, and then the whole thing was slid onto the old chassis rail sides and located and tacked. Then I added whatever else was needed to make a proper crossmember.
Then the gap at the top and bottom between the old chassis and the new crossmember was filled with a plate of just the right size, and then I overdid it a bit and covered each of them with a longer plate that overlapped the chassis and crossmember! And then there was the towing mount and everything else. Small problem from all this welding: the thing bowed a bit.
I suspect if you use these long "tabs" for a proper crossmember that will help in locating on the horizontal, and then you can figure out where you want it on the vertical. The biggest problem I had with all of this is actually getting everything in exactly the right place for a body tub to mount correctly.
At any given point in time, somewhere in the world someone is working on a Land-Rover.
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