Sorry for your loss...
IMHO, build this rifle for yourself. It's one thing to do a project you and your father discussed and always wanted to do, it's quite another to build a "in honor of" or tribute rifle. To me that puts too much emotional strain on the build, especially if things don't go as planned or turnout the way you want. I'm sure he would want you to have fun with it, so do.
While there are some meaning to some decorations found on longrifles, you specifically asked about hex marks on the barrel.
I only know of one "hex" mark, and that is a Cross or an X placed on the bottom flat of the barrel, under the rear sight. Some say that the stars and or dots you sometimes find on a longrifle's muzzle are hex marks.
The tail I've heard is that barrels that shot untrue were unbreeched and washed out in a fast moving stream. Then the hex mark was placed on the barrel to prevent the demons or evil spirits from reentering the barrel. I do not know if this was ever done to new barrels.
18th and early 19th Century maker John Phillip Beck would sign the underside of his barrels in Latin, "J Deo Beck". In English that means with God's help, Beck. The Latin phrase "INRI" is also found on the bottom of some of his barrels. This comes from from the sign on the cross the Romans placed over Christ's head, "IÄ“sus NazarÄ“nus, RÄ“x IÅ«daeÅrum" or in English, "Jesus of Nazareth King of the Jews."
Personally I would not consider Becks hidden marks as "hex" marks. To me they are more of measure of respect by a pious maker.
North Carolina Gillespie rifles are found with Turkey tracks and pine bough decorations all over the barrels on some rifles.