That's the wrong stringer to use on a longbow. It's designed for recurves. Get a double pocket stringer, or better yet, learn how to use the push-pull method of stringing. (Agree with evg, would only recommend for a longbow.) Quite easy to do.
I wondered about that, not having much experience with longbows, and even less experience with heavy draw weight longbows. I thought the stringers with friction pads were specifically designed for longbows, since in my limited experience the double pocket stringers have a tendency to slip off of the upper limb tip. The limb tips on a straight limbed longbow point slightly downwards when stringing the bow with a stringer, which makes it more likely for the upper limb tip to slip out of the stringer. At least that's been my experience; do you have a solution for that?
OTOH, the friction pads make it necessary to exert more force to bend the bow sufficiently to string it, compared with a double pocket stringer, which isn't a great problem with a 45# bow, but would be much more significant with a 75# bow. BTW, I have even more of a problem using the friction pad stringers on recurves than longbows, because the recurved limbs put the friction pad at an angle to the limb which makes it more difficult to get enough friction to stick. So if they aren't designed for longbows, then I wonder what they are designed for?
I've never had a problem stringing a lightweight longbow using the push pull method, but it becomes more difficult as the draw weight increases.
I suppose the best method for me to use to string a heavy weight longbow would be the step through method. While I would never use this method for a recurve, due to the risk of limb twist, I notice my longbow shooting buddies using it all the time, since I suppose there is little to no risk of limb twist with a longbow.
On second thought, I think I’ll just leave the stringing (and shooting) of heavy draw weight longbows to those more qualified (and younger) than I am.