The 2.5 is essentially the same engine as the good old 2.3 Lima.
Just a little more stroke. As far as reliability, it's great. Very common to see em go 300,000 miles before a rebuild if maintained properly.
As far as performance upgrades, you're looking at an upgraded air intake, free flowing exhaust. Those will improve fuel economy, and give you some noticeable gains, hp wise. But, like you said, you don't like the idea of a loud truck. Oh, by the way, you already have a header stock.
Beyond that, you can upgrade to an aftermarket computer, from a few different manufacturers, and I've heard pretty good feedback on em, but have not tried one myself.
As far as the resistor installation to fool your MAF, don't waste your time, and $.12 (yes the $40 kits you buyfrom EvilBay are nothing more than a resistor from TandyShack.)
The biggest limiting factor you're going to encounter is the auto tranny, it just doesn't let the engine rev up into the RPM ranger you need to be in to get the most from your motor. Also, you might want to check you diff (can be checked on the tag inside your driver's door, and decoded using your owner's manual.)
About the best combination is the 4.10 (which is what I have).
Anything lower, and you will suffer off the line, and since you really can't go much faster than you can with the 4.10's, you may want to consider swapping if you don't have em already.
All in all, you're not going to get more than about 15 HP from all those mods, and that's about all you're going to get from a 2.5, without some serious $$$ and major build up.
In the case that that's what you want down the road, the most popular routes to take are swapping in a Turbo 2.3 from a 87-88 T-Bird (and a few others it appeared in), or a good old fashioned 5.0 V8 Swap.
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