Im using a 1/2" phenolic spacer and I could tell a difference.
First off the upper intake is MUCH cooler even after running hard or for a long time. This keeps more heat in the lower intake, which is being water cooled so that helps.
As for power, I noticed more power down low. Carbs have been doing it for a long time and it does work, quite inexpensively I should add.
Another benefit is I can change my valve cover gaskets without removing the upper intake. There is enough room on the passenger side to use a 3/8 ratchet with a short extension. On the drivers side you can use the 3/8 ratchet on every bolt except the top middle. There I use a 1/4 ratchet with a 3" long socket. It's a tight fit but I would rather take a little longer and save my upper to lower intake gasket.
if your ever gonna go with roller rockers you will need taller valve covers. They might hit your intake dpending on what type of intake you have. I have the 1 inch spacer and it works pretty well. (not only reason for the spacer of course)....you might wanna check hood clearance before you purchase one. Some cars are different than others. thats just my experience with them.
I have rollers and I used that taller valve covers. I did have to modify the bracket that holds the linkage for the throttle cable to the t-body to get it to clear. I shold have just gone with the spacer at that time and it would have cleared ok. I picked up spacer wednesday and I will put it on this weekend. Thanks for y'alls help.
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89 GT-gt40 ported heads with manley valves,cobra upr and lwr,E303 cam,1.7 Rollers ,pulleys,80mm Mass Air,190lph pump, MSD Ignition, 3.73's,Frp Axle Guirdle, tremec 5 speed,billet flywheel, Ford Racing Headers (Jet-Hot coated),Off Road X pipe, 24lb injectors, 70mm tbody,subframe connectors, aluminum driveshaft,HPM Megabite Jr. Lowers , 1" intake spacer,lakewood 50/50 shocks and 70/30 struts,Wild Rides Battle Boxes,2 stage dyno tuned chip,vortech S trim blwr @ 6psi
spacer between upper and lower intake 
