If you can get your hands on some hemp thread and twist it into the plies it'll compensate. I actually had to ask a spinner to make me some because it is hard to find.
Dunno if you do this sort of thing, so please forgive if I sound like I am patronizing, but whenever I stitch 18th century I have found this useful;
Take your span of linen and about six inches extra. Take another span of hemp. Both are unwaxed.
Clamp the ends in a bench vice or knot them on a hook. Either manually or with a low speed drill and eyelet bit, unwrap the first span counterclockwise until it starts to reverse and then pin it, trying to keep it from twisting. Do the same with the second thread. Don't worry about the three or four inches on the end yet, saving that for tapering and waxing those ends.
Now un pin the first thread, stretch them both taught and twist them up in their original direction. It'll be a touchy-feely thing to figure out how tight is tight for the twist.
Wax the living $#@* outta the newly interwoven thread. Whether you you have COAD or beeswax, load up the thread, take no prisoners, and burnish it in with your fingers if you are a tough guy or a leather pad if you're a New England Pats fan ( Steelers here, so I am not supposed to ever forgive you for what you did to us....anyway...). Leave the ends raw so you can taper them and wax them up good also when satisfied with the taper.
My master always used 2 individual plies hemp and a span of one to three ply linen, unless he was using a 5 ply linen then he would only use one ply hemp. He'd threaten the strap if I snapped the stitch tight, always urging me to pull firmly to close. If you are using COAD it'll actually impregnate the stitch unless you have a hole the size of the Holland Tunnel and are using those big harness needles.
Again, forgive me if you already do this sort of thing and I am beneath your skill curve. Give that a shot and let us all know...K?
