When you look under your fender you see that the available real estate has been encroached by some large protruding fasteners. You can remove some of them and replace them with stainless steel flat head and pan head socket type fasteners from:
www.boltdepot.com/product.aspx?cc=25&cs=82&cm=19&cd=1318The list of possible replacement fasteners goes like this:
Item # C
Product #6698
Metric socket flat head screws, Stainless A-2, 8mm x 1.25 x 16mm
Quantity: 4 pieces
Price: $0.54
Subtotal: $2.16
Item # B
Product #6702
Metric socket flat head screws, Stainless A-2, 8mm x 1.25 x 35mm
Quantity: 4 pieces
Price: $0.73
Subtotal: $2.92
Item #A
Product #6657
Metric socket button head screws, Stainless A-2, 8mm x 1.25 x 25mm
Quantity: 4 pieces
Price: $0.96
Subtotal: $3.84
Item #D Welded Nut
Shorten slightly with 4" grinder
Overmold (blend a hump) with JB Weld Epoxy
Paint with silver touch up paint
Item #E (not shown) Mounting bolt for Right Buddy Peg
Shorten slightly with 4" grinder
Paint with black touch up paint
You will need to buy a HSS countersink to countersink the flat headed screws to be nearly flush to the sheet metal that they will retain. Close to flush, but not completely flush. You do want some metal there to clamp up to, right?
Go slowly here == making the countersink too deep & large means you won't hold the sheet metal in place and that is a classic "bad thing" when trying to hold stuff together with flat headed fasteners.
The main fender mounting bolts get replaced with shallow pan head fasteners instead of flat heads. Why pan heads? You got these protector bumps in the sheet metal and all you really need to do is get shorter than the bumps. Pan heads are stronger than flat heads and the fender is a structural element that sees strong vertical vibrational forces and potential tire/fender impact forces. It needs more fastener strength than the horizontal mounted gingerbread stuff that we flat headed for width clearance.
Note that the item #A fender mounting holes here aren't simple holes, they are slots. If you need to shift the fender a bit to avoid a rub this is the time to do it.
Now, do you REALLY need to do all this removal and replacement of fasteners? It depends on how big your oversize tire is and where it runs when installed on your rim.
If you were rational and picked a 150/80 bike tire you likely won't have to do much at all. If you were moderate and picked a 145-15 VW tire you wouldn't have to do near as much. If you plan to put a 175/65R15 Continental on as your next experimental tire after you do yourself a chain conversion -- heck, you may need to flush mount everything you can possibly flush mount.
As always, this information is for theoretical discussion only -- nobody is actually ever going to do any of this silly stuff. We all got better sense that that, naturally.
Oldfeller
PS Buy the type of countersink with a single straight cutting edge that is easily resharpened with a standard bench grinder -- you will likely have to resharpen your countersink edge at least once while removing all that metal.