The narrowbands simply put "switch" they will go to 8-900mv at cruise, idle and so on. Yea 900 mv is richer than 800, but you cant tune wot with them by any means. They are constantly switching to provide the "stock" pcm the info it needs to maintain 14.7:1 afr on a "stock" engine. I would assume with the money you have in a 408, and you have efi-live that you wouldnt want to melt it down. Just put it on hold till you get a wideband.
If your tuner put you on a sd tune how come the maf is reporting data. It should be failed. (cant look at the tune on my phone)