Thank you everyone for your contributions. I can verify that McReall's scan size fix works for my 320U even at 300DPI with no black lines observed, under Win8.1 x64. I have added the modified driver to the main post in case hex editing is too troublesome.
I won't be installing Windows 10 until...
I realized that as well... the drivers came straight from a Windows XP x64 disc, so I'm guessing that when the driver is compiled for 64-bit systems the options to scan other dimensions were somehow left out. (I don't have a XP x64 machine or VM to test this...)
This is just a FYI as I had just managed to get my own Acer 320U scanner up and running in Win8.1 x64. This driver also works in Win7 to Win10!
The same drivers work for the following abandoned Acer/BenQ scanner models:
310U/620U
320U/340U/620U+/640U
620UT+/640UT
1240UT/5100U
640BT/640BU...