I had a similar query earlier in the year. In the end I installed the irons on each side and then used some circular clamps to attach some garden stakes to each side. I then did an ACCURATE inside measurement of the upholstery kit I already had and adjusted the distance between each corresponding iron. Satisfied with that I then took the bows that I had and did a match up. Like you, I was totally in the dark. In the end two fitted perfectly, the wider bow needed a little sanding to fit into the iron and about a quarter of an inch trimmed off the length. The last bow was shaped right but too short for a totally snug fit. Meant I needed to dowell fit a shaped extension piece to that one rod. A bit of a drama, but I ended up with a top iron and wood kit that fitted the pre-made upholstery kit.
Cheers
John