I'm going to ask the same question a different way... maybe that will help.

I have a 1998 GMC Z71.  At 199,250 miles I changed the engine out from
a 305 to
a 350 (crate - new in the box).  That was 22K miles ago.  The truck has
been
running great until about 3 months ago when I noticed it getting hard to
start after
coming in from offshore (2 week hitches).  (By "hard to start"... I mean I
would turn
the key, the engine would turn over... but not crank up.  I would have to do
this for
several minutes until it cranked)  Now...Today / currently... I'm having to
(turn the
key in attempts to "crank" the engine) - for about 15 minutes before the
engine will
"crank" - "fire up" - "run".   
Now, here's the kicker.  This problem ONLY happens when the truck sits for
a few
days.  If I crank and drive the truck daily - it "fires" right up every time.  
("fires up"...
meaning, the engine cranks up allowing me to drive the truck) - Driven
daily, it
never gives me any problem.  Cranks every time.  57 times in a row if I ask
it to...  
Just when it sits up for a few days does it give me this problem.
More info... I have no problems with the battery - it's hot and ready.  My
starter and
my fuel pump (which I hear when I turn the key on - humming like it's
supposed to)
was changed not long ago and are working as designed.  Plugs, cap, rotor
and
wires were changed when the engine was installed (22K miles ago).
So, when I'm attempting to crank the truck after it's been sitting for a few
days...
"first attempts" - it acts like it has no fuel.  Like the fuel line is empty.
eventually, it
will hick up and cough a little.  Very little smoke from the tail pipe if any...
and it's
black when it does.  It has only back fired once since all this started.  It's
acting like
it's flooded at this point... but then again, once it cranks and I drive it - it will
crank
and run every time.  Until I let it sit for a few days.  Then, it's more of the
same.
So what do you guys think?  What's happening here?  Why is my truck
doing this
to me?