I’m running a 372 CID small block (340 block, 4.06” bore, 3.58” stroke) with a 4 speed manual and 3.91’s out back. The gear vendors turns my rear ratio into a 3.0ish gear when engaged and allows me to cruise comfortably on the freeway.
I would not run 3.23’s with my combo but I’m running a fairly large solid flat tappet cam and an LD340 intake with the plenum divider partially removed so it likes a bit of RPM. Your 408 should make more torque so 3.23’s around town may not be that bad.
I am still very happy with the unit and chose this path as I did not want to modify my cross member or trans tunnel to install an overdrive. My install only required some minor grinding to the gear vendor adaptor and a few small dimples in the trans tunnel.
