Tunnel ram plenum spacer

I would be very careful about adding a bunch of plenum volume on a manifold like that.

It’s not a 1970’s design.

You can keep adding volume on the dyno and it will make more power

In the car it will be a pig off the brake or the pedal if you foot brake it. It will be slow off the gear change(s).

You really need to test that in the car.