The 260 doesn't have the "bow out" in the tunnel for the catalytic converter. I'm not sure which 280's have that transmission tunnel change. I'm also not sure how that changes the carpet floor piece (just under the seat, right?) Newark makes kits for both early and late 280Z styles also. I should know - we have a totally stock '75 right there in the shop... Crap, we also have a 77-78 on the lift - I should look at that...
Speaking of vinyl on the trans tunnel, I've always REALLY liked the "small diamond pattern" that the 260's have. I've also always thought the "large diamond pattern" of the 240's was lame... I know,,, It's just me,,,