'70 and later had the vent located further to the left so your housing isn't a '70. You need lines for a '65-'69 B-body.
Yep, ran into this exact same issue with my '70 Roadrunner 8-3/4" rear end. '70 is a change-over year between the two locations for the brass "splitter" which are about 2" or so different & you could have either one on a '70.
The company where I got my brake lines (maybe Inline?) just sent me the "other set" of rear brake lines no charge. Maybe the guys you bought yours from will do that? If not, take some measurements & I will see if my "extra set" of rear brake lines happen to be the ones you need (stainless fyi).
And now that I look at your picture a little bit closer, I see that you can easily cut & re-flare the passenger line you have shown in the picture. The problem will be that your driver side line is too short.