Before you do any of this, you need to do a Mass Air Conversion. You're car can't handle cam/head/intake mods appropriately and will run all screwed up unless you swap from speed density to mass air. I had to do that, too. You need a Mass Air Sensor, the harness for it, and a 89-93 computer that matches your car (5.0, auto or 5 spd). You will need to swap from your headers to Ford Racing's GT40 P specific headers.The plug placement and orientation on a P head is different than any other 302 head, and requires headers with different runners. Don't listen to people who try and hack in other headers, it's a pain, you have to disform your headers, and it doesn't work right. 70mm TB is right on target, that's the one I will be getting ASAP. Stock injectors will run fine, my car ran rich on the dyno before I tuned it to a neutral A/F ratio. Porting and polishing is definitely worth it, I wish I had gotten mine done before I bolted them in, but money and time were both tight, so I didn't. It supports better flow, which means more power. There's alot that goes into this other than just the heads, cam, and intake. you need new head bolts, new gaskets, roller rockers, possibly new valve covers, an adjustable fuel pressure regulator, a new timing chain, hardened pushrods, new valve springs setup for atleast .550" lift, new valves, guides, retainers, and seals, and once you do it, lots of other weak points are gonna show up- IE- the day I got the car back, the oil pump died, then the radiator let go, now the tranny is going. It's a never ending cycle, but it's worth it. Any more questions, post them up, and don't let me discourage you, I'm just filling you in on some of the hard spots I hit.
