Joined: Jan 2013
Posts: 1,339
Likes: 7
From: Westfield, MA
Bikes: 1983 Univega Gran Turismo, 1970 Schwinn Super Sport, 2001 Univega Modo Vincere, Self-Built Nashbar Touring, 1974 Peugeot U08, 1974 Atala Grand Prix, 1986 Ross Mt. Hood, 80's Maruishi MT-18
You could use a frame mounted bag also. You would probably lose the use of your bottle cages, so would need a hydration pack or handlebar mounted bottle cages. The rest of your questions, I can't help you with. I'm not familiar with carbon Treks and never rode the GAP/C&O.