I rememmber the factory hole at 6.5" which means that a 6.5" speaker would fall through as the mounting diameter is ~5 7/8". A 6.5" driver is cable of some bass, but I don't know of any 6.5" drivers that have more than 5mm of excursion so they don't move much air. There are some 8" drivers capable of 9+mm xmax which is a vast improvement over most affordable 6.5" drivers.
Top vs. bottom mount - do whatever is easier. I think either way you'll need an adapter plate.
