FI & Man Trans running-on

[Vogt Family]

Every time I drive my grandfather's truck ('89 F-250, 351, 5-speed)
there is a condition that bugs me.  When I back off the accelerator and
put in the clutch to shift, the engine RPM stays the about the same 1-2
sec before dropping, and it doesn't seem to drop at a very predictable
rate.  Now a while back I drove somebody else's FI manual transmission
car (but for the life of me I can't remember what it was) and it seemed
about the same.  This condition is very annoying to me because it makes
it difficult to achieve a smooth shift by letting the clutch out as
engine RPM matches with the next gear.  Any clues on what causes it and
what I could do to avoid it when I build my next project?  I suspect it
has something to do with the engine management being more built for an
automatic, where such a condition would not be noticable, but what do I
know.

Birken