.32-40 reamers are common - just not with the modern crowd. And rebarreling with an octagon is no big deal by those who do it all the time. But before you jump in, there might be a couple other things to consider.
1. A #3 is a pretty hefty barrel for a lever rifle.
2. Most .32-40s are going on singleshots and most singlshots are shooting very heavy (long) bullets because they have no restrictions on OAL and they are generally shooting benchrest targets at 200 yds.
3. By rebarreling, you disqualify your rifle for NRA lever silhouette competition. This game is driving the values of lever action rifles these days, esp. the older ones. You can have it rebored (but from .30 to .32 probably won't go, but .30 to .38-55 would), or you can have it relined. This will maintain more of your rifle's value (and its patina it has one).
Relining with a TJ's liner in the correct twist for the bullets you want to shoot (200 gr lead?) is the only way I would go if I was determined to have a .32 from a .30. You will have to go elsewhere to get a good estimate on what a good twist rate should be. Lots of smiths can, and do, do this. But you might take a poke down at the BPCR forum here or better, some place like marlinowners.com or SteelCity.com, or a few other places where lever guns, and 19th century rifles are a bit more "normal". This is a stainless and plastic bolt world for the most part.
