None of the convertibles had seamless frames. They were all 2-piece frames with the large x-member in the middle. As far as I know all other models could have been built with seamless frames. I know I have used them with and without the extra HT mounts, and I have a 56 210 4-door sedan with a seamless frame.an older local guy claims that they was usually under convertibles more than h/t's and post. but said they did put them in the leiter to also.