You can easily single load with a controlled round feed action too, just need to scallop the extractor to pop over the rim. If I was building a custom tomorrow I'd opt for an after market Mauser- Granite Mountain, Satterlee, etc. If on a budget, a Winchester Model 70, one of the FN South Carolina ones, and in stainless. And if after something different, completely change gears and go with a No.1.
Reasons I'd pick the Model 70,
-5 rounds down in the mag, standard chamberings, typically one better than most
-Controlled round feed (works just fine for single feeding, too)
-Three position, best and most copied safety in the business right from the get go
-Accomodates anything up to .470 Capstick without alteration (depending on the mag box you started with)
-Lengths in ultra short (WSSM), short, long, and extra long (only a mag box)
-Best resale out of all the normal affordable base actions for a custom, as let's face it, most of us let them go someday to build the next curiousity
You're likely going to fit some M70 features to a custom rifle anyhow, especially the safety, so why not start with one.