The ratio of overdrive gear vendors provide is not really worth it. I drove a buddies 727 with gear vendors and was disappointed with the rpm drop when it was engaged.
If you really want to enjoy your 408, swap in a built 200R4 with a high stall lock-up torque converter. It's like having a 5 speed automatic. The 200r4's first gear is .25 lower than a 727's, and the overdrive is .67 to 1.
There's a night and day difference between the two ways to add overdrive to your old Mopar. If you drove your car with each set-up installed, you wouldn't ever consider the gear vendors.