I thought the 340's 275 hp rating was realistic and not conservative as with some other engines? The 316 hp in the above mention build was somewhat believable with an optimistic dyno but getting to 400 hp with a mild cam, intake and headers is a stretch.
NHRA back in the day rated the 340 4V at 325HP. This was based on 1/4 ET's, vehicle weight, gears and tires, using comparisons of thousands of runs and removal of engines for dyno tests.
The 340 six pack was rated at I believe I remember 360HP.