Greetings Corvette Bill.
This sounds like it will need to be fabbed. Since both ends of the running boards bolt to the fenders, and the fenders are different from 46 to 47, (and as you discovered the rib count is different), you'll have the make something work.
If you're going to keep the 47 rear fenders, maybe you can just clean up the splice. If you go with rear 41-46 rear fenders, maybe you can find the rear part of that year boards and splice them in.
Just my opinion.
Good luck1
~ Victor
1941 3/4-Ton Pickup (in process). Read about it
in the DITY Gallery1955 Grumman Kurbside "Doughboy" 235/3 on tree w/ OD
1957 3100 - moved on
1959 C4500 Short Bus "Magic Bus" - moved on
1959 G3800 1 Ton Dually "Chief" - moved on
1958 C4400 Viking "Thor" ~
moved on to fellow Bolter 
