I never really gave a lot of thought to just the slave mount, since 460's and diesels have an integral boss.
Rear counter shows Bob Allen Ford in Overland Park KS having one.
You might RFP and see how dear they think it is.
Eta:
https://www.rearcounter.com/E4TZ-7A544-A-parts395455.html
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.