Pick up a set of split port heads. That will be the single option you have for any noticeable power increase without FI. Those and the windstar intake can be picked up at your local salvage yard. Then work on suspension, gears, and tires.
Forced induction is the only option for any decent power increase. Hell even the m90 will bring you up to split port levels. That being said those cars are dogs. 140hp is awful, paired with the stock 2.73 gears and, yeah, not good. I had one, miserable car. I'd go for a handling setup that gets it riding like it's on rails, and save the extra money to go with a better sixxer or pick up any v8, as any mustang v8 will be a big difference over a single port 3.8. I believe the only mustang power plant that made less power was the NA 4 banger at 108hp.
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