Distance between on a 550 is going to be smaller than the 750, why do you ask about a 550 or 750? The odds are low they have std brackets that fit a certain point then adapter tubes to the bike mount points. It is more likely they used model specific mount frames that attached to the bike. Use of a secondary bracket like I mentioned at first is going to be weaker and subject to vibration loosening the bracket. You can cut and cap the existing tubing or cut and change the angles with some cutting and splicing in tubing that redirects the mount brackets to your mount points. Splicing the same way you would repair a frame with a section of tubing that fits inside the existing tubing to your tubing and that then splices the end with the mount point relocated to fit your bike. Like Frank says if you can weld tubing or braze tubing you should be able to adapt the brackets it comes with if it is from a DOHC 750 such as the one in the brochure. The brochure shows the rear turn signals hanging from a relocation bracket on the support rail from the rear rack.
Many if not most bikes unless designed for the addition of rear bags in mind of the 70s vintage had the signals on the sides of the bike requiring the wiring extended on the signals and signals relocated because they were around 6 inches behind the shock mount. Lots of later bikes moved the signals to stalks beside the tail lamp.
David
