how are uoi intending to mouunt it?
Delta has a 2 strut one.. struts are shorter.. P clamp up the seat stays.

Seat post racks are a common workaround for FS mountain bikes..
Spaniel Ear rack bags and beam racks with some side support loops on them,
will allow expansion space.

the alternate if you have a hard tail, is adding stand off spacers
on the left lower mount,
and making the rack splay wider, to clear the disc caliper,
for symmetry, the same may be done to the right side ..
Calhouns cycle shop in MN offers hex standoffs with a stud in one end.
your rack bolt goes in the other end..

a longer bolt and tube spacer may do OK, Perhaps upgrade
to 8.8 marked bolts then, for their higher strength steel.