I think what is overlooked is not the raw speed or physical ability, but that to run a good 400m is a skill based around ability to relax at a very high proportion of maximal speed, and it's not one that many middle-distance runners would have acquired.
I don't think I ever broke 55 for the handful of 400m races I ran, but I had done 4x300 in 39/40 sec, 4x400m in 57.xx and could always throw in a 25 at the end of something like 10-12 200m with 200m jog where most were 27/28.
I would guess I could have run high :53.x if I'd acquired the skill.
I ran 1:57.5; 4:20; and 8:45 for 800m/1 mile/3000m
Of course there are outliers at either end. Really speedy 400m guys that drop off a lot, but can still run a decent mile, but slow in relation to their 400/800. Then at the other end the one time British record-holder for 6 miles, Martin Hyman was almost completely devoid of speed, and didn't break 2 mins for 880 yards, but ran a 4:06 mile.
I'd say for a median type guy who is 50/50 fast/slow twitch a 54/55 capability should get the job done