You can use some parts from a certificated airplane in your project, but those parts will not count towards the 51% amateur built number you need for experimental amateur built status. Here are the relevant passages from AC20-27G:
(Page 11) The use of used or salvaged assemblies (for example, landing gear, hor. stab., and engine mount) from type-certificated aircraft is permitted, a long as they are in a condition for safe operation. However--You should contact your local FAA MIDO or FSDO prior to using a major assembly or sub assembly, such as wings, fuselage, or tail assembly from a type-certificated aircraft. [a local DAR would serve as the representative of the FAA in most cases]
(Page 12) The practice of performing alterations, repairs, and rebuilding of previously type-certificated aircraft for the purpose of obtaining an experimental amateur-built airworthiness certificate is not authorized under Section 21.191(g).
It is my guess that the project as described in the first post would not qualify as amateur built. Ultimately this is an FAAA decision, but I would be sure to get the OK from them before I spent much money on this.