Definately go aftermarket. Kinda odd you want sports car handling while keeping it stock though!? I cant say I've had any experience with those kits, but im sure you could check out some various magazines to get some performance numbers. I just somewhat recently put in a Mustang II front suspension setup in my 65, but havent yet been able to drive it since I decided to rip it all apart.
I decided to go with a manual R&P with my setup, since its a pretty light car. 67's are a bit heavier, but I still wouldnt see you having much of a problem with it. Another reason I went with manual is so I didnt have to deal with a power steering pump/hoses and all that stuff.