Your original route is in part covering similar ground to my 2010 Lejog though I went via Arran. I bypassed Dumfries but that meant crossing the A75 dual carriageway near Collin. I had no problem at all and would do it again, though another CTC member has cautioned against crossing here. Look on Streetview and decide for yourself.
I used parts of routes suggested by Phil Hosley's in his book "Land's End to John O'Groats". On the way to Sanquhar where we stayed the night, I cycled on some quieter roads than you've chosen. The following morning from Sanquhar I took the A76 because the previous afternoon I missed the right turn into Sanqhuar and cycled an unnecessary circular loop. Though the unclassified road to the south west of the River Nith is featured in Phil Horsley's book, from Sanquhar the A74 has my vote.
I didn't go through Kilmarnock but your plan might give a slightly less complicated route through Irvine than mine.
Day 11 Carlisle to Sanquhar
http://www.bikemap.net/route/627860#Day12 Sanquhar to Lochranza
http://www.bikemap.net/route/628392#Michael's 2010 LEJOG
http://www.cycle-endtoend.org.uk/index. ... &Itemid=93
The views at the top are def worth it, plus you get to free-wheel downhill for a good few miles after it!
