I think the general consensus is that you need to try these things on (and load them up if possible) to get a better idea. I know some rucks (older vulcan) came in different back lengths and some come with adjustable back padding etc but I don't have specifics on the sabre 75 other than the Fformat thingy that can be shaped. Some people seem more able to get on with rucks that don't fit 100% and some rucks can be really weird and seem made to fit quasimodo so I really would recommend trying before buying.
I've got a 35L Lowe Alpine (no adjust) and a size 3 80+20L Bergy Vulcan, both fit fine (6 footer) may not so fine after a huge march in them but OK for about 10 miles.
Cheers,
Alan