Saddle fit help & long pasterns

[kudosirony]

I am working with an OTTB, Thunder. Being such, he needs what I would guess a narrow to narrow-med saddle. While I'm looking for a suitable saddle for him (I do not own him, simply a horse I am working for the owner), my current saddle does not fit him. It's weird, the back sits nicely on him, but the front sinks down too low (I've been wanting to reflock it though, so perhaps there's the issue). I have a Stubben Scandica Medium-wide which fits Cracker like a dream. It's ridiculous, we've found the tool to a beautiful partnership. But I digress. I need something that will at least help my saddle from being far too wide for the guy. I use a Dover wool half pad, but this doesn't do too much. Bless him, he doesn't let the ill fitting saddle from trying his hardest to work with me, but it's definitely not making it easier on him since we do dressage and he really needs that freedom. Did I mention I'm trying to find a decent narrow to narrow-medium saddle for him? I just need to keep throwing that out there for when people go on and on about how nothing can replace a well fitting saddle.

Also, he has long pasterns. I thought I saw something in the conformation critique about long pasterns, but I can't find it. So, long pasterns - what do I need to look out for, be cautious about, etc.