Looking at that one pic with the fender on it looks like its hitting on the front? Like the curve of the fender is tighter than the circumference of the tire, Plus the fender mount strap looks to have been tweaked a bit over the years. Might want to make some tweaks to the fender to see if you can get a wider curve, I've been able to achieve that by using a 220lb "Markus press"......standing on the top of the fender while its on the ground :laugh:
I took a comparison shot between a 4.00x6" Knobby (what Ruttman used) and the 5.30x4.50 6" Universal. Both aired up to 20psi mounted different wheels but same widths. You can see there is a height difference, but like I said not really that much, It may be enough worst case, but I think you should be able to get that 5.30 in there with some tweaks after seeing your pics. Seems like I have seen it done with the universal treads in the past on the Ruttmans, of course they did briefly make the rounded universal in the 4.00x6" size too, think the MTD bikes may have used them on the fronts if I remember correctly.