I would stay with the 351 and build it up, lots of stuff out there for it. If you are going to build to 500 hp Dave is right, it would be a good ideal to put at least a 6 point cage in, If it is done right it can also look real nice. Here is a site with some cool things for the cougar, got any pics?
http://www.totalcontrolproducts.com/main.html