The 2 January 1969 ordering info shows the following base prices: 6 cyl coupe $2638, 6 cyl convert $2852, 8 cyl coupe $2727, 8 cylinder convert $2941.
I spoke too soon-shouldn't do this at work anyway.
The last 6 cylinder at the old price is N569621 12D build. The next 6 cyl is N571751 at the new price, 01D. Really strange is N570985 an 8 cyl coupe with a base price of $2743.
Cars shipped after 1 Jan 1969 were at the new base price.