They used to be the ONLY "performance" head you could get for a small block Ford. Thanks to this NEW car that came out in '79 and got a little popular in the 80's (Fox Mustang, for those in Rio Linda, Ca. Lol!) there are a million better heads out there. I would think that any of the Explorer GT40 heads would SMOKE a 69/70 Windsor head. They cost about the same as those you are being offered for $250, also.
Unless you are doing a "points" restoration on a 69/70 Mustang or you are a Ford parts "collector", I would just find a pair of the GT40's from an Explorer.
My opinion.