8 3/4 U Bolts

Does anyone know the thickness (Diameter) 1/2" or 5/8" etc... and length of u bolts when adding an 8 3/4 housing to stock springs on a 67 Dart GT? And should they also have the top rounded part flattened?
Just curious as I can have a spring shop make up new ones, but they want to know how the dimensions. Bolt thickness, threads (fine or course) and length and width.
All I know is standard size is 3"W x 7" or 8"L
I'm not sure if they can flatten the curve part as per OEM. I have no way to order Originals online so have to rely on a well known business who does brakes, suspension work.
I am trying to convert from a 7 1/4 rear to 8 3/4.
Appreciate any help.