Imho, assembly is different from manufacturing. With assembly the parts and components used have already undergone r&d and are already "proven to work" or perform as expected. Using AMC as an example, if the donor car used is a nissan patrol, theoretically the AMC jeeps/hammers are expected to be as reliable and perform similarly to a patrol. Now, if everything AMC uses were designed and manufactured in-house from wipers to engines and suspension, reliability and performance could be worse or better.

But still, It can all be done, it all boils down to money, lots of it.