I’m not sure if you’re still interested in this option but the 20 degree hill you mentioned would be perfect for some acceleration development work. A typical acceleration development workout would be running all out for 10-30m with about 2-3 minutes rest between reps.
If you wanted to only run 60-70% as you said, then you’d need to run much longer to get any real benefit out of it and even then I don’t know if the benefits would be very specific to 400m running.
ELITETRACK Founder