I will be using HE on my 3.0 (some day when $ is comming in rather than just going out). Will I still need any of these things?
The flapper boxes and hot air tubes off my 2.0?
The rubber seals off the 2.0? Are they different on the /6 tin?
The diffusers that hang down under the firewall?

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I kept the air foils for my 6. Rubber seals should be the same.

If you are going to put heat exchangers on the six, then you need the flapper boxes. Also keep the rubber seals and the plastic flaps.

The rest is four cylinder only parts.

Thanks, I guess that means I can make a project out of cleaning up the flapper boxes

/6 flapper valves have a big hole in them ..............

Yea, but they are hard to find, and the /4 flappers will work. And if you check PET, the /4 part number supercedes the /6 part number for the flapper boxes.