I mocked up the install of the 3-link upper mount yesterday. After moving some wiring, and the fuel line/filter(!), out of the way, I have a better view of what needs to be "trimmed" from the old 4-link mount. The cut line is marked with tape in the photo below...

2019-06-12 19.52.32.jpg

After trimming, I plan to bolt it all into place using your tip about make a bolt plate, inserted through the open end of the horizontal 2"x3" box tube. I am not a welder but there's a fab business close by and I may ask them to weld the old and new brackets together. That would make the mount stronger, I assume.