Three key words in the original post: 90% street driven.
The comment that a single plane intake has better fuel distribution is incorrect, because you have 7 cyls interfering with the cyl drawing mixture, compared to 3 cyl with a dual plane.
The Edel RPM manifold really changed the goal posts with the SP v DP intake debate. I have seen dyno tests where the RPM was better everywhere in the rpm range over a SP intake. Not what was expected, but the numbers don't lie.
If it was my engine, I would use the RPM.
In a 408, the RPM will probably peak at about 6200-6300 rpm. Because a DP manifold usually idles better & has more vac than a SP, you could increase duration by about 5* for more HP without sacrificing street manners. Also agree with solid roller lifters, either with a sol cam or hyd.
