I believe the 340 is the 255 that you're talking about. The 340 is just the model # that flows 255lph. They used to call it the 307 but have upgraded some of the mechanical parts (I've been told). Here is a link that compares some of the different walbro models...hope this helps
http://home.pacbell.net/sfnelson/walbro242_307_340-2.html
The MKIV Supra intank pumps have more out than the 255lph Walbro's.
I called Magnafuel about there 4303 pro tuner electric pump
and he said that uses the same pump as there outlaw pumps.
With the belt drive flow/psi goes up as the rpm/demand goes up,
with an electric pump as the psi goes up the flow goes down and
the amp draw goes up loading the charging system.