Your not gonna want to hear this but the answer to your question is to get a saddle that fits. Will adding thicker socks make shoes that are too tight fit your feet ? Sure you can add a thicker wither area pad to lift the front but guess where your weight goes ? Right on the hip lower spine area. Seems most of the behavior problems I have dealt with on Walkers and other big gaited breeds is cause by people putting QH saddles on them.
You know, I kind of have to chime in because I had the same problem and didn't even know it for quite a while. I have a Missouri Fox Trotter and was using a FQHB saddle (the only good saddle I owned at the time) and she would sometimes give a small buck (more of a kick really, not a full out buck) when she got excited. Not enough to unseat me so I just thought it was her high energy level (she is a high energy horse).
But one day after a long trail ride (over 12 miles) I realized she had a small bulge on her spine near the back of the saddle. I never had the front end touch her withers because the pad was cut-out there, but the back end of my saddle was bottoming out on her spine. The flare of the bars was simply too wide for her and the saddle was contacting her spine when it shouldn't be.
Anyway, I am riding her in a narrower saddle now (still a QH saddle, but either regular or semi QH bars instead of full) and that little bucking thing she did has basically gone away.
Hopefully you won't have that problem, but I thought I would mention it because Joe's post hit home with me.
That's not to say your saddle won't fit...... but if any strange bumps or behavior issues crop up, keep in mind it could be the saddle. Being that you already own this saddle, I would go ahead and try a pad with a cut-out in the wither area. I use an Impact Gel or a 5-Star, but most high quality wool felt pads have a wither cut-out.