I see 32v Mustang cobra's for under 7K all day long. Or a GT of 5k + $2k for a used blower setup.
whole premise is how cheap you can build a v-6 car? you ignore how much it will cost to upgrade the fuel, and ignition system, somone did mention rearend, THen add all the hidden cost, if you are going through all the trouble to keep the new motor fuel injected, or just carb it, both routes still cost more. With the buyers market there is right now for 10 year old v-8 cars why waste the time on swapping when you could be driving, add all the cost of pain in the ass garage time instead of driving your car around.
even with a donor car you will still have odds and ends that nickle and dime you out of pocket.