I asked our lead assayer about this - He used to work for Barrick and other large mining companies in Nevada.
For large chunks of silver, they normally drill "pin samples" in a grid pattern with a hollow drill to get a representative sample to assay. Grid size depended on the chunk. He said yes, the silver assay can vary all over the piece with a chunk this large.
One of my classmates used to be a silver miner in North Idaho. He and a buddy found a chunk of Native Silver about 2' long, maybe 1-150 lbs. They cut it up in chunks large enough to fit in a lunchbox, and hauled it home one piece at a time. The piece he brought into class looked like a slab of bacon, cut off about 3x6" at one end, and tapered down to a dull point 8" away. Just small enough to fit in a lunchbox.