I got great help from my friend, Teresa who pointed out the obvious that I could not see - or hear, as the case may be. The Waltham ribbing is divided into two patterns: 1/3/1 and 4/2/4. In other words, you knit 1, purl 3, knit 1 and then purl 4, knit 2, purl 4, and so on. DOH! I didn't see this at first. Interestingly, Teresa is a musician. She heard a rhythm in this stitch pattern to which I was completely deaf. After she pointed it out, my knitting simply sang. Thank you, Teresa!
Now on to my modifications. I extended the sleeve cap by 2.5 inches because I'm narrowing my crossback width. I bet that just made your eyes cross. Let me show you instead.
The original pattern calls for a 23.5-inch crossback at the 60-inch size. My crossback is 18 inches - that 5.5-inch difference would make the sweater way too large for me in the shoulders and upper back. Remember that a well-fitting crossback is key to a well-fitting sweater. So I need to make this mod.
My plan is to make the upper body of my sweater 5 inches narrower; I will talk about this more when I start knitting the back. But for now, I need to worry about making the sleeves fit in to the sweater body, so I need to make the sleeve cap longer. Instead of a 2-inch sleeve cap, I'm making mine 4.5 inches long to accommodate the narrower upper back.
What would happen if I didn't extend the sleeve cap? The sleeve just wouldn't fit into the body; I'd literally be missing 2.5 inches on each sleeve.
So, that's the story with my sleeves. Hopefully I'll finish the second one tonight and can give them both a much-needed bath and blocking. Then I'm on to my next worry: weirdo waistshaping for my weirdo body. Details after I noodle this out.
If you have trouble with your Waltham, send me a message. Or visit the Ravelry group. I'd be happy to help or find someone who can.