What type of filler door you ask... are you ready for this? How about a 37 Ford passenger car filler neck and cap.
Just spent the day widening my stepside bed 4 inches today. Yeah, I know, I am wierd

I did this for more rear tire clearance and just so it looks better: This will hide the indentations on the back of the cab & the stepside beds are actually narrower than the front width of the truck at the front fenders, to me that is backwards.
I could have bought the wider fenders, but to me those just don't look right.
I will post some pictures as things progress a bit further.
John