I personally dont care for much domestic. I definitely donr believe in supporting anything that it not modern and high tech in a modern vehicle. Leave the pushrod motors where they belong, in GM crap and in relics of a bygone era. Now as for the northstart talk, very cool. I am a big fan of the northstar and it's smaller version, the 4.0 aurora V8. I know of a local yard that will sell the whole engine/transmission combo with harness and cluster for $2500.
You will need a RWD transmission that bolts up, and the only ones that do come off the 60degree GM V6's in the camaros or trucks. Any RWD GM with a 2.8 3.1 or 3.4 should have a transmission thats works. Or atleast have the right bolt pattern. Please tell me you would only consider a manual trans. Problem is, I believe those vehicles only came with 4-spd manuals and 3spd autos.
You might actually be able to use just the bell housing and then find a BorgWarner 5 spd that would bolt to that BH.
I think that would be about the only appropriate domestic engine to consider for a 240. If you would go V6 make sure it is a 60degree V6 so it is even-firing and balanced. 90degree V6's suck and are only a compromise so they can be assembled alongside V8's which are optimally balanced at 90degrees.
I havent priced many Q engines, but why not go for a VH41DE or VH45DE (both Q45 engines). I have no idea what transmission would fit, but i would start with the VG engine series since it's also a V design, it may share the same bolt patern on the bellhousing.
One weird thing about the Q motor is that the intakecmes in on the side and has funny runners that snake their way around and finally make it to the heads. Not sure if it would clear the 240 hood, but it should, and regardless, if I was doing it, I'd make a custom manifold using about 6-8inch runners into a sheet metal aluminum plenum for much more top end power. The Q is heavy so Nissan focused around low end grunt to get that big beast up and going in a 240 you probably couldnt stall it if you popped the clutch at idle.

A friend and I had brief plans to do a VH41DETT in his RHD 97 240 but plans were shelved in favor of a much simpler S14 SR20 swap, but know knows..... maybe some day you'll see something like that come from me.. if some doesnt beat me to it...... hehe.
Sean D.