So I scrounged around the various junk piles and pulled out a 4x4 transfercase crossmember out of a WW2 era Dodge ambulance (everybody has one of those right?). It kinda looked like a model T rear spring perch, but was way bigger. I cut a piece out of the middle and tried it on for size.
It is going to span between the lower a-arms and the spring will be mounted above the crossmember.
Here it is mocked up, centered under the spring.