Re: 5.8L ?
Here's what you may want to try, and I'm sure I'll forget a couple of things along the way:
1) Use your 90 engine harness, and sensors. When you start pulling everything out of the engine bay, LABEL EVERYTHING. 3" masking tape and a pen work well.
2) Convert the 351 to the speed density setup for 1990. You may have to route some of the vacuum hoses differently for the MAP sensor, and the 351 vac. ports from the manifold.
3) Forget the MAF. It is inline to the intake tube between the airbox, and throttle body. Use your 1990 intake tube if possible. The MAP is on the firewall next to the A/C unit.
4) You shouldn't need to do any cutting to the firewall for anything at all. The EEC module is located under the kick panel on the driver's side of your vehicle. Pull the interior trim out under the dash. You'll see the module. It's about 6" square, and maybe, 3" thick. You'll need to take the mounting screw off the wire harness from the engine side of the EEC (I think it's a 10mm head). Pull the harness off, and take the EEC module out through the interior. MAKE SURE that you reinstall the orange rubber seal. It's the only protection you have from the elements.
5) make sure you use the 351 firing order when you reinstall the plug wires. 1990 302's were the original 302 firing order.
6) Just take your time, and try to think through your situation, you will be allright. Try to find a 1990 351 donor vehicle (truck or Bronco). you can pick alot of the production stuff off it to aid you with your project. You may even want to take a few polaroid pictures to use as a reference.