I've printed and sold 40x60 prints, modestly cropped (mainly straightening the horizon), made with my D700. And my D300. And my D200 (10 mp). And my D70 (6 mp). I have also printed and sold several panos, 30x90 give or take with the D700 and the D300 -

The notion that you need more megapixels for larger prints is basically a myth. There is no magic done to the many iPhone 6 camera images put on billboards that you see all over town, and that camera is only 8 mp. And the billboards are huge.

FYI, for a 40x60 you only need to print it to 32 ppi, so all you "need" is 1280x1920, or roughly 2.5 mp. So a 40x60 printed with a 6 mp image looks pretty good, and using a 12 mp image allows you to crop.

This is an accurate and great explanation of why:

http://www.photokaboom.com/photography/learn/printing/resolution/1_which_resolution_print_size_viewing_distance.htm

Try reading the small print in a newspaper at a distance of 5 ft. You can't. But it's easy when the newspaper is only 16" away.

The only time you need more resolution on a large print is when you are expecting photographers - who will walk up to the large print, take out their loupe, and start examining the individual pixels.

Think Billboard. I have a 30x40 hanging behind the reception desk at the tennis center where I work taken with an 8 mpx. Olympus with a sensor smaller than either Nikon or Canon's crop sensor. It looks great from 8 ft. away, and would anyone want to view it any closer? Actually you can move to within inches of the print before you see any softening of the print. I have both the D700 and D800 and I like the 700 better. I feel with the less mpx., less sensor nodes, larger nodes and space between them it handles noise really well. I know on the "bench" tests show the 800 and even the D7100 to handle noise better but it has been my experience that the D700 does just as well or better.