I don't think that will be strong enough. It will put too much strain on the header bolts and this bracket will bend under load. The exhaust has to be solid so there is no movement at all. If at all possible you want to keep these mounting brackets with a straight pull not with unsupported 90 degree bends.
You'd be better off getting a piece of fairly thin plate steel and making a triangle shaped bracket and then using a longer bolt with spacer to mount to existing mount on bike.
If you want to use the type of bracket you are showning than you should weld a triangle guset piece in both 90 degree corners. That will make it more solid.
I've had both Sporster & Road Kings mounted on my 77. Currently using the Road Kings. They both mount differently. On the Sporster's I had small tabs welded onto the rear of the bikes frame so that I would have a straight pull to the muffler. On the Road Kings I ran a thick piece of aluminum from the fender mount bolt to the muffler & then made another clamp for the front of the muffler that fits into the stock mounting location. You have to make these mounts as strong as the orginal exhaust was otherwise you will have problems.