Don't know if all things are the same for a 68, but Vicki's library has the 67 "Service Manual Supplement" which has a diagram of the heater hose routing (in the "Custom Air Conditioner" chapter)
WITH YOUR HELP !, trying to put back together a factory 1967 convertible 400/TH400, softray glass, pw, ps, pb, ac, console, clock, safeguard speedo, remote and visor mirror(s), hood and trunk lamps, and custom trim options.(Rally II rims and hood tach had been added). THANK YOU ALL for any help, opinions, and/or advice. I'M GONNA NEED IT !