Hi Jet2us,
I think it would be hard to squeeze 13 pallets into the 737-300. 13pallets was what we loaded onto the B707 freighter which was a much longer airplane. (The stretchedDC-8F was even longer and could carry something like 18pallets).
The 737 freighter can carry 8 P1P (88x125ins) pallets but for a QC you may loose one pallet due to galley and toilets so you may be left with only 7 positions.
HTH
Anil