I think I have it !
I decided to download the driver and re-install/overwrite it.
It is the exact same one - same version.
The original was installed off the CD that came with the printer.
My first custom size print worked - yay!
There is a bit of 'overspray' or maybe it's old ink on the rollers that is on the clean white border area - too bad ! But that is a different issue altogether.
The overspray you refer to could be a head strike as the paper might have a bit of curl that release's right at the end of the print.
What I do for my 3800 when I cut sheets off rolls is to leave a couple of extra inch's at the end, so once the printer is finished laying down ink, the curl that release's will happen after the print head has finished, so there won't be a head strike, just don't tell the printer about the extra paper length.