Using the GE motor in na-t's is actually reasonably common. This guy proved they can go fast (9.45et on stock bottom I believe):
http://www.boostwerx.com/index.html
Pricing you have above seems spot on. I still want one someday.
I'm worried about how rich the guy say's it runs until warm. Sounds like washing the cylinders type of rich. And if so, they gas is mixing with oil, and that is never good for anything..