'65-'70 Mustang floor pans will fit in '67-'70 Cougars,..but they are not the same, different ribbing and drain plug locations. As far as who to buy the pans from,...thickest material and cleanest stampings would be my choice and preferably from a US manufacture.
US made is the way to go. but if you do buy ones that are thinner material it shouldn't matter just as long as you treat it right to last a long time. POR-15, epoxy with sealer, undercoating, etc...I put thinner gauge floorpans in my 68 and they went in easy and are pretty stout actually.