For a degreaser alot of people here, including myself, use Simple Green. It's biodegradable and doesn't take the paint off the (red) block as much as say "Gunk" spray.

I've used BG44K (BG Products) before with great results. Cleans the whole fuel system, intake valves, and will remove carbon on the pistons. This is powerful stuff that you drop in the tank... ALso, when using any type of fuel injector cleaner (Techron/BG44K/Redline stuff/etc...) it's best to put it in a few hundred miles before your next oil change. This stuff (esp. BG44K) will dilute your oil a little which will increase wear if you run with it for the next 3000+ miles.

As far as manifold removal is concerned, you may be opening a can of worms by taking off the exhaust manifold. If you do decide to do so, spray the 8 nuts and the exhaust pipe nuts with PB Blaster every day for a week (or more). Doing this will greatly reduce your chances of breaking a stud which is a PITA to fix. Also, get new nuts. Check out the FAQ for details. I just did what you are planning to do on my '80 GT. Didn't remove the exhaust mani and was still able to get behind it with the Simple Green and toothbrush. I did have the intake off which was a big help, easy to remove.

Good luck,
Bean
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'80 242GT 92k, '94 945T 123k