If I were building a CUSTOM, I think it would be a Mannlicher stocked Mini Mauser in 6.5 Grendel.. I have a Mini Howa in 6.5G and it is kind of clunky, I have DIP bottom metal and bobbed the mag.
6.5G Mini Howa, handi little bugger though
If my pockets were a little deeper it might be a full stocked Sako L-461 in 25-45. I already have a Savage 10 in 25-204 so know the little rounds are very capable.
25-204, 100gr Partition at 2850fps