What is the best setting to use for recording structure scan to make a side scan mosaic? 455 or 800?

This is something you really need to trial for yourself on the water you are mapping. In general, I prefer 455Khz for all but the shallowest water (and shortest ranges).

Ever since LSS-1 was changed to LSS-2, I have much better results with 455. My LSS-1 will do well in 800 on short range but for mosaic you want the longer possible range. So stick with 455, this is usually better even in shallow water due to the more open cone. (I currently use a LSS-1 on my smalest boat, a TotalScan on the midle one and StructureScan 3D on the largest.)