use of spacers will not be a problem, I've done it for decades.
Using common sense, ie bolt long enough to go all the way into the threaded part, a good solid spacer, proper tightness and a little blue loctite and you'll be good to go.
Its easy to source those solid aluminum spacers, they have diff hole sizes, so get the right one for your bolts, and then you can cut them to the right length using a hack saw.
The advantage with one spacer is less parts to lose if you have to take off the rack, and a bit easier installation.
On one of my bikes, I did have to mount my Tara using the "wrong" bottom mount on the rack (designed for fender mount I guess), but even though in theory it isnt as strong as the other holes, Ive toured extensively on that bike with a heavy ish front load on very bumpy roads in other countries and all has been fine over 4 1/2 months touring time.
Last edited by djb; 03-26-19 at 08:17 AM.