400 ft/lb ain't stockish- for a 292 inline!
The problem I see w/building a 300 HP NA engine then turning it into a blown engine, is the cam and pistons will automatically have to be replaced. If you're gonna do a blown engine, I would do it w/o any NA stops along the way.
The inline 7 main bearing engine is stout, no doubt. But I would line up a couple cores, just the same.