IN MY OPINION, upgrading the brakes on a 68 Valiant would increase the value for me. Especially if you go with front K-H discs. (SBP) The matching rear drums would be 10" x 1-3/4". (Summit carries the front rotors) Those disc brakes were optional in 68. Going with newer 73-76 a-body discs and a LBP rear has the advantage of better wheel selection. And 65-72 a-body 8-3/4 rears are not rare. I see several at every large Mopar show/swap I attend.
It's not like a 68 Valiant is highly collectible where a potential buyer wants to see a build sheet and is worried about matching numbers. Don't get me wrong, I love Valiants and wish I had one.