Mike ~ Have you tried using a shorter lens? When I work in close on either 8x10 or 11x14, I think in terms of what focal length will focus at a reasonable extension. This was done with a Cooke process lens (about 10 inches, I think) that isn't even rated for 8x10. On 8x10, I usually use a 6 inch lens. Coverage is seldom an issue that close, though I always check the corners...
Now the exposure times at small stops at that extension in subdued light are a feat...