Hope these pics are of use to you and if not I will have to take some better ones for you. Kevin is comparing this section of his convert frame to my hardtop frame.
Did yours still have all those crossmember mounting holes in the same place as mine ? I still think all they did from GM was add on the extra material for boxing in. I can see why yours would have a different crossmember for the convert frame. Adding those parts would change the mounting location for your frame compared to the hardtop frame.
These pictures are great and I believe confirms our suspisions that all they did was add the piece. Crossmember mounting holes are visible on the passenger side on mine and look close. I didn't get to inspecting mine today closer to see how badly scaled it is inside but if not too bad I'll cut them off and use them on the HT frame. Trans mount between vert and ht look different. Got a start on the seat? Keep coming home with stuff like that and the wife's not going to let you come out to play ! lol