Just be aware of road works on and around the M74 at Hamilton, it's not too bad apart from peak time morning and late afternoon so avoid that time if you can. They same goes really for both the M74 and M8 around Glasgow, can be a bit of a car park at peak times.
My own preference of route would be to stay on the M74 rather than cut off onto the 73 and then merge with the M8 just past Glasgow city centre on the top side of the Kingston Bridge. Not much in it either way just my own preference.
As others have said, off the M8, over the Erskine Bridge then onto A82. Never been up to Drymen but the route to the Erskine Bridge is pretty straight forward.