I have a chance to buy a set of GT40P heads (assembled) for $500. Should I take the heads and run or should I look at a differnet set of heads for my 91 LX with a 5.0? I haven't done much to the car, yet, but I have put in a Hypertech performance chip, MSD billet distributor, MSD 6AL ignition, a Holley 190 lph fuel pump and a K&N filter. Also, I have been told that I need a set of headers made for the GT40P heads, is this true? Will the stock lower intake fit/work with the GT40P heads? If anyone can help me give me a shout.

I have a set of the P heads on my car now.  You said your car is an Lx.  The stock intake will fit.  You will need a set of motorsport gt40P headers. the spark plug angle is different and the header tubes extend straight out for mor clearance mine are ceramic coated and were $349 and the stainless steel are about $250.  I know the P heads are good for 12.'s off motor and 11's of nitrous they flow good.  I recommend milling them down .030 I did and it brought my compression up to 9.8:1 and I have .529 lift and have no clearance problems. back when my car was bone stock I added the "P" heads milled .030 a cobra intake and 1.7 roller rockers and gained 2.3 seconds on quarter mile time. with an Lx stick with 3.73 and those few mods  are good for 12.80's and eibach drag springs and drag radials.  but 500 bucks is only good if the are new or just barely used you can buy new ones for just under $600  

Thanks. Now where would I find the headers?  

you can find the headers at a ford MOTORSPORT dealer.  Duthler Ford In Grand Rapids Mi can get them for you.  (616) 956-5511 ask for parts and your ready to rock and roll  

i wouldnt bother with p heads svae your money for some trick flows or afr. if you do go with the p's you will need headers, which im sure you can find on ebay along with the heads probably for less than $500.  

Hey I just picked up a set from a J.Y for 262.00 off of a 98 Explorer. Thank you for the info on the cam an rollers. Was worried about clearence so you have eased my mind. Cobra heads go through one more decking than the truck heads. My question is did you have to have your intake ports milled for the intake with the .030 taken off or did the Cobra intake bolt straight up to your heads? Thanks for any info you can lay on me relating to this head swap. I have a 89 LX that is sweet but aint fast enough for me.  

$500 ?? Don't do it...
Honestly, take it from someone who had P-heads..(you will spend $$ on new wire everytime they hit the exaust manifold because the plug angles are different( had to finally use chevy wires, I KNOW...), the spark plugs are different, exhause header will be different, need I say more?) . save your money for other heads. you can probably find a decent set of heads out there for a few hundred buck more. check out the companies out there and the local car shows and swapmeets to get an idead of what really works... Take time to invest in a good set and not just settle because they are cheap? Good heads are not cheap, unless you got a friend that wants to buy full out race heads and need to unload his current aluminum heads quickly. Try to unload a set of GT40P heads...You'll have better luck finding a snowshovel in Cancun. Better than stock but not worth the $500.  
Heads and cam are the secret to a good running stang. So you will pay $500 now and then $1200 later.. so that is $1700 for the same results??? I know I did it..