From what I have read and heard, it is a waste of money. I am sure someone here will have a different opinion, but lets hear from someone who has actually used one.
It depends on the intake manifold and rpm range of the motor . On a motor with just a few bolt ons and and a stock manifold it isnt worth anything. If you had an aftermarket manifold and a decently modified motor with a cam that will alow the motor to make hp in the upper rpm range you would see some bennefit from the the extra plenum volume the spacer gives you.
I just finished a gt 40 tubular paint and polish.........89 5.0 it pretty much stock besides the basic bolt ons .....and just wondering if i should add a spacer when i do the install ..........