2000 Silverado misfire

Have truck I bought from a friend that
had sat up for a few years. I drove it a
couple of weeks and it ran fine other
than the 4wd inconsistently working.
About 2 weeks ago it got to where it
was hard to start but would crank
and run fine after pumping the gas
pedal a couple of times then one day
I was driving it around the end of
field, I let off the gas to take it out of
4wd and the truck died. I finally got it
to crank back up and got it home
(about a quarter of a mile) but it was
missing terribly and would not keep
running if you let off the gas. I've got
a p0300 (random multiple misfire)
code and consistently am getting a
p0101(maf) code. The things I have
done so far are change fuel filter,
changes plugs and wires, check that
each cylinder is getting fire, clean air
filter, clean maf sensor, check all
injectors (resistance, for power, and
to make sure all are clean and not
leaking), replaced fuel pressure
regulator, replaced crank sensor,
replaced map sensor, replaced cpv
valve, replaced intake manifold
gasket and cleaned intake manifold,
checked egr valve. I have fuel
pressure of about 50 lbs but rapidly
bleeds off after the truck is switched
off, my vacuum is very low as in 8-10
inches at 1000-1200 rpm. I have
pretty much run out of ideas. Also
fuel pump was replaced the day
before I bought the truck but it was a
very cheap one, could that possibly
be my problem? With egr removed
the truck will stay running but it is still
running very rough. I was thinking the
catalytic converters might be broken
loose and stopped up but with upper
o2 sensors removed it still will not run
at idle.