The problem with buying those smaller mirrors now is that they are often tied to LRE's to raise the price.

Kinda like how any mechanical part is suddenly worth a small fortune when its tied into a hemi e-body.....

As an aside, over 20 yrs ago I put a set of those mirrors on a '76 Club Cab I had. The backing plates were MIA. I used carriage bolts with big fender washers. The outside looked a little 'trick' with the smooth heads of the bolts, and the fender washers provided plenty of support.