I just remembered that the easy way to locate the fenders over the wheels would be to position the fender mount flanges 25-1/16 inches from the centerline of the frame. That comes from the bed dimensions. 50 inches internal width plus 16 gauge (1/16" thick) bedsides.

Fab a couple of brackets that fasten to the frame with angles positioned above to catch at least two of the fender mounting bolts. Those 2nd from the bottom fender bolt holes are 5-3/8 above the bottom of the bedsides, which are pretty close to the top flange of the frame where it's flat.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.