Buick 65,
I'm not sure what you mean exactly by "curved back bow". It should not have a curve towards the front though, it should be straight across, side to side, if that's what you mean. There are good illustrations of the bows in the Buick Body Service manual.
Adjusting the top frame: be prepared to be totally frustrated and challenged. Getting the top frame in the correct position was one of the most difficult for me to do. Adjusted and readjusted dozens of times until I felt comfortable. You will need to get it right before the convertible top fabric and window go on. Any adjustments afterwards will create wrinkles in the fabric. There are detailed instructions in the Buick Service Manual. There are numerous adjustments to make so that it lines up with the window glass and the front windshield. Read and study the manual.
Good luck,
John