The GTX in 1972 got the 440. The 400 in the Roadrunner is a HP engine, I personally would build the 400 into a stroker instead of putting a 440 in. The early build date 400's had the thicker webbing in the mains that is desirable.
http://www.440source.com/blockinfo.htm
440 vs 400 thread:
http://www.forabodiesonly.com/mopar/archive/index.php/t-131600.html