i think u cud mound upright ( meaning at 90 degrees rite) as long as the fins on the heatsink also run vertical. amps in dad car rite now at about 80 degrees and no problems but we have fans
correct me if i am wrong, but i think that it doh matter at wat angle u mount an amo, as long as it sturdy and not shaking up or sliding around and once heat can get out of it and air can flow freely in and around it