You can change to 14" wheels without any mods as long as the bolt pattern is correct, having said that I think the FC Drums have dimples or the like that may stop you.
To run front discs there is a myriad of options including most pre commodore holden stub axles plus you have two types of aftermarket drop stubs. Do a search and read around to see what options might be best for you.
HQ etc diffs would be too wide, the easiest option for a HR disc front end swap is the HR diff, pretty much a bolt on swap.
BTW, I removed your post on the Resto board to avoid any double ups, most users will see all the new posts so you won't miss out