Well, of course a blower will make a lot more power, but it will also cost alot more too. Since you have a Convt, an intercooled prochager running 9lbs should put you in the high to mid 13's, and with a bigger MAF, gears, and slicks, your going to be closing in on 13.0's, maybe high 12's. But I think you really would need a good fuel system and 11lbs of boost to get 12's on street tires.
Now the head swap is also another great choice, especially if you want to stay N/A. Now, I am not sure if the 98's pistons are the same as the 96/97's, but when you add the PI heads to the 96/97, you are also bumping compression up to about 10:1, where as the 99+ GT's are running about 9.4:1 because of diferent pistons. So, assuming the 98 have the same pistons as the 96/97's, you should see at least a 40HP gain, but maybe more, especially if you do some very light porting and add a 3 or 5 angle vave job. Because of the higher comprssion ratio, you really should be making more power the a 99+ GT w/ the same mods. But, a 10:1 compression ratio is not too blower friendly, so if you are doing the head swap, make sure you do not want to put a blower on w/o making other mods.
You could of course change and upgrade the pistons if you plan on adding a blower in the future, of you could have the heads worked to lower the compression. Hope this heps.
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1997 Thunderbird 4.6L: Dynomax exhaust, Lightning 80mm MAF with 7" powerstack cone filter, B&M 24,000GVW trans cooler, Inject-Tech chip, KVR Brake upgrade. 15.2 @ 91. Not bad for a 3800lb auto 4.6L
No Longer own: 91 LX 5.0L 5 spd
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