""The 1976-1977-1978 blocks for the 400 and the 440 are a thinwall casting design. As such they shouldn't be overbored more than .020".""

If you check out the tech section here on sonic testing you will find that the thin wall therory has been pretty much proved wrong, and also the lower end strength around the mains (tech section from 440source) is considerably better on the later 70's blocks than on earlier blocks. Also some block ribbing improvements made later too.