I have a 7STW and really enjoy shooting and hunting with it. The only down side to the STW (IMO) is that quality brass isn't really available.
If you build a custom rifle
with all the bells and whistles you may want to consider a cartridge that has readily available high quality brass.
My longest 7STW kill, which would be short range on this forum, was @ 465yds. The 162gr A-Max made a decisive kill and the shot was not all that difficult with this flat shooting set-up. I like the 7STW.
Good luck with your project