Both can be built.
6 bolts were the earlier and probably over engineered. In my opinion, 7 bolts were brought back in line (over engineering brought back in line with target requirement of Mitsubishi).
What this means is they probably recognised that:
- Rods were big and heavy. Could be improved by lightening.
- ports were too big and reduced to over velocity
- head studs were too big
- eng mount to transmission mount design change may be related to either efficiencies to labor or structural improvements (bell housing support).
Wouldn't be great if we actually had a chance to get the thinking and calculations behind why Mitsubishi engineers did what they did (performance or manufacturing influence).
End of the day, most have the option of building whatever 4g they have. Most influential preference is based on the chassis they own. I know many wouldn't go out of their way to convert.
With that in mind, I like 6 bolts because I own Galants.