This is JMO but 600hp on the juice is pushing it a bit for the trans if it don't have all the good stuff in it. I had a all stock body 68 like yourself and it went 3375lb with me in it and I go 180lb. P/G's like a light car best around 3000 or less. If I was going to do more street then strip I would stay with what I had get a 3500 stall and a sportsman brake put in it and have fun. As I said before its JMO. I had the very same set up that I just told you and went 11.79 in the 1/4.