mikek,
Chambers can and do vary quite a bit depending upon who chambered it so caution is always in order. Your description of what is occurring is rather puzzling as Rel 19 does not have a reputation for burning dirty, but it obviously is in your case. It is possible that the particular lot of powder you have may be unusual or it simply is not working well in the combination you have. Obviously, something else is in order.
The reason I recommend staying off the lands with the heavier bullets is that pressure signs may appear before reaching the velocity window which the cartridge is capable of achieving when the bullet is into the lands, at least that has been my experience.
You will want to talk with the gunsmith who chambered the barrel or can make a chamber cast to see what it looks like, but you will still need to taylor your loads to what it will accept. I would also recommend speaking with tech support at Alliant Powder.
http://www.alliantpowder.com/about/contact.php about the Rel 19 burning dirty. The brass you are using could also affect the case capacity. (308 Win brass, especially GI brass typically has less case capacity than 260 Rem brass or reformed 243 or 7-08 brass for example.) The people at Sierra Bullets are also good to talk to ( 1.800.223.8799).