If you want to go NA than sell your I4 and get an LS.
I've owned both and have been around half a dozen of them and that is my best advice for going NA. You can spend all that money for the extra 30whp (if your lucky) and you'll be lucky to go as fast as a STOCK LS. Seeing how an LS is about $500 more than a stock I4, it is advicable that you simply sell the 4 and get the LS.
So in other words.....TURBO.
If i had my I4 I would turbo it by now. It is a relatively simple process since there is so much space to work with. The engine layout is perfect for a turbo setup and fuel upgrades are cheaper than on a V6.
You can get a custom turbo manifold made pretty easily from your local turbo shop and the rest of the setup isn't that tough either.
Buy Maximum Boost by Corky Bell and read it, it will teach you alot of things. Asking about turbo on this site is not something I would recommend unless you have read that book. If you haven't, most of the answers you'll get won't make sense to you.
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