So I eventually got it worked out and it turned out the plate was perfect for the job. This is a heavy 1" plate, one of many that I salvaged from a former employers scrap bin. These plates were used as jig assemblies in the manufacture and repair of jet engines. Every once in a while they would clean out their storage, and discard items from contracts that were to old to ever be revisited. Aluminum and steel were fair game for employees. Titanium and titanium alloys were worth to much, even as scrap, and were out of bounds. For all I know, this jig was used to produce parts for the Avro Arrow (the most advanced jet fighter of it's time). It was built at this plant, although all associated technology was supposed to have been destroyed when it was canceled.
Here the slot for belt clearance is cut out and I added four more holes (ha, ha) to bolt it to the head assembly.
Here I temporarily installed it on the head to check the fit. It will get a 1 1/2" angle iron frame on the underside for further support.
And here is the scribbled sketch I finally came up with after I got my head straight yesterday.