The ST-7400 is not 'natively' compatible with any RD other than the 740? series.
Another concern is the FD. The 6400 series had 2 versions of FD, the later 6401 is designed to work brifters, the earlier 6400 was for downtube shifters and needed more cable pull.
The 6400 pivot design was similar to the FD-A550, that used what Shimano called forward push, which moved the cage forward as well as laterally. This meant that more cable pull was needed to index properly.
And even with the FD-6401, the cable attachment arm was designed to handle both downtube shifter and brifters. It had 2 position on the arm where the cable could be attached, one on the shorter side, closer to the pivot for brifters.use if needed.