Can Ag be read as Pb in spectroanalysis?
ie. are there overlapping spectra lines?

I'm unfamiliar with the physics involved, but it looks like the NIST
has the data you want here.  Since I am not doing any analysis, I am
posting this as a comment rather than answer.

Silver:
http://physics.nist.gov/cgi-bin/Elements/elInfo.pl?element=47&context=noframes

Lead:
http://physics.nist.gov/cgi-bin/Elements/elInfo.pl?element=82&context=noframes

Click on "lines" to get atomic spectrum information.

silver (Ag) has an atomic mass of 107.868
and lead (Pb) has an atomic mass of 207.2

so if you are using a plain Mass Spectrography technique you would not
confuse the 2 as they would appear as different lines.

granted that there might be some isotopes of silver (atomic number 47)
and lead (Atomic number 82)that exists in nature, but isotopes differ
in atomic mass by very minimal units i.e. the most by 1 or 2, so you
should not see any over-lapping lines between lead and silver.

Nope.  No interference between Pb and Ag lines.  The common spectral
lines are as follows (optical wavelengths given in nanometers):
Pb: 220.3 280.2, 283.3, 386.3, 405.8
Ag: 328.1, 338.3, 529.9, 546.6
These different wavelengths correspond to different frequencies,
colors and energies of light and are easily distinguished from
eachother.  In simpler terms, since none of the above numbers
characteristic of Pb are the same for Ag, there is no interference.
Ref: http://www.lesker.com/cfdocs/newweb/framesets/Frameset_Instrumentation.cfm?Navigation=/cfdocs/newweb/Process_Instruments/OES_Endpoint/Spectral_lines_Species.cfm

There do exist intereferences between other optical spectra though. 
For instance the 220.3 nm line inPb can interfere with Co, Cu, Na, Ni,
Fe, Sn, and Zn.  Also, the 338.2 nm line in Ag can interfere with As.
Ref: http://www.glow-discharge.com/spectral_interferences.htm

hope this helps.  I can't submit an answer since google is full of
answerers.