Are the frobnt ends (fender headlight housing and lower valance) the same on the 240, 260 and 280?

No. The 240 and early 260's have front turn signals below the bumper on each corner. The later models have solid lower corners with the turn signals in the grill area. I'm not sure about the center section.

headlight buckets, yes.

valance pieces, no. the valance changed size & shape as the bumpers grew larger and the turn signals moved.

Are the fenders the same exact size and shape?

Yes. I'm basing this answer on the VB catalog where they list replacement fenders as the same part# for 1970-1978.

Don't forget to mention differing metal and fiberglass headlight buckets!

Are the lower valences inter changeable on the 240 &280z?

I wonder if you could put a 240z lower valence on a 280z. I have seen a 240z with a 280z lower valence on this site somewhere.