64k is the per-model polygon limit. furthermore, it's not advisable to pack everything that belongs to the level in one (model) file. 51k polygons my not be much at all for an entire level, but way too much if it's all in a single file. for speed reasons, single model files should not exceed 5k polygons, best rendering speed has been achieved with models between 1k and 5k (can't find the chart right now). speaking of terrains, use chunked terrains with LOD steps, which breaks up the large terrain into small pieces. you can easily do that by script. chack the manual for chunk_size or similar.
