This was my solution to the MI EGR problem. I got mine three or so years ago off ebay. If you're lookin for a MI remember that you're looking for a boat part so searching "marine intake vortec 5.7L" or such may not turn up many if any hits. You are looking for a Mercury/Mercruiser Magnum 305 or 350 intake year 2000+. Mine came off of a 2005 Mercruiser 305. Boat yards take them off boats to sell to other boat owners not necessarily truck guys so keep in mind while you're searching. But with regards to the performance question it's an improvement but won't make a difference for towing. Eliminating the flow restrictions is only beneficial at high rpm which isn't really an applicable tow scenario. But it's easy to tune, you can get Bosch EV14 (popular on Ford trucks) injectors to fit it, and you won't have the spider issues to worry about. So no question a better intake than stock. But for performance it's a lot of money for virtually no gain.
In my opinion plenum spacers are snake oil. You'll spend a lot of money, have the future headache of other than stock dimensioning on the motor top, and not gain a thing for the effort. But that's just my opinion having seen them but never tried myself.
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