Stock 400 was a hydraulic (flat-tappet) cam that was pretty mild and I think it spec'd a 10W40.
I would go with an HDEO in 15W40. Depends on your budget, there are some boutique oils specifically designed for flat-tappet cams in older cars, but yours is not one that puts extreme pressure on the cam, so the HDEO would work well. Delo, Rotella, whatever, find one on sale...