A F550 ford uses different wheels than our GM trucks, the ford wheels have a lot thicker center and they use two piece flange nuts, the flange swivels on the nut.

Again, I would still use the Clamp plates on the GM 19.5" 8 lug (and 10 lug) dual wheels. There are a few step vans with the 19.5" 8 lug wheels still here where I work and they have always had the clamp plates on them.

In this tech tip http://www.stovebolt.com/techtips/wheels/ in the "Overview" section is a link to the Accuride wheel catalog. http://www.accuridecorp.com/files/2012/10/Accuride-Wheels-Product-Catalog-Summer-2011.pdf
On page 27 is all the info you need from the company that made the wheels you're using.
Quote
wheel size 19.5 x 6.00 Part number 29015 Hub-Piloted Dual-Mounting 8-Hole, 6½" Bolt Circle, 4.56" Bore (3/4 , 1 Ton, Chevrolet or GMC Typical)
Use Clamping Plate w/90° Cone Nuts; Clamping Plate General Motors P/N 472536

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-