Well, this thread ought to get interesting in short order so let me chime in.
If it were me, I wouldn't sell to my friend for that unless he had the knowledge, resources (think $$) and intentions of trying to preserve a rare car. There are three things that I certain of: 1) I can promise that you likely spend more $$ than that car will be worth done in the end unless you can and will do the majority of the work yourself, but you can take solace in the fact that you will have a very rare fish on your hands in the end; and 2) If you sell this to your friend you will likely not come across another one; and finally, 3) The fact of the matter is that A bodies are not E bodies and this is not 5+ years ago. The market and economy remains soft, which may have been the impetus for you being able to even get your hands on the car for what I assume was less than the 5k your friend mentions paying you to take the car off your hands in the first place.
You are correct about the cost of original manifolds. Repops are not offered to my knowledge. There are members out here that have them, but they are commanding a pretty penny, and most are squirreling them away for something of their own. Also, not sure how readily available the BB bellhousing is either, but I know other members know with more BB know-how will perhaps chime in on that.
Just my $.02, but since you do not have the numbers matching block, there is no reason not to put some TTI headers on a 383 and call it day. Like I said before do your best within your abilities and budget to keep the car as OG as possible, but there will have to be some deviations unless you are patient enough to wait for opportunity to knock on correct, rare parts, and when they finally show up, expect they will cost you.
The first order of business would be to pull the back seat and see if there is an intact buildsheet back there. If it's there, set it aside and start a record in a binder on the car. I would attempt to get the backstory on the car from the previous owner and put that stuff in a three ring binder and start gathering parts and/or buy a parts car. Take a ton of pictures and keep a record of what you do to the car, records of parts bought, and $$ spent.
IMHO, if you friend is into Darts, let him find a Dart (there are plenty out there, and he can do with them what he will). There are PLENTY of people on here that are salivating over your find, they just have not come out and expressed it like BB Dave has. There are also many talented members that have the patience, $$, knowledge and some parts lying around to treat this car to a proper resto if you felt compelled to send the car to a good home. Additionally, if you choose to hang on to it, there are helpful members on here that can and will help you get through any sticking points on the car. You could also thank them by starting a resto thread in the resto section, and I am sure this car would have quite a few people that would subscribe and like to see how it progresses. I also suspect someone will decode the fender tag properly in short order now that we can read it better.
Thanks for sharing your find with others like me that are happy to know these cars ARE still out there. I assume by your handle this is not your first Mopar, I just want to offer my thoughts. There is a great restoration log that a guy named Jeff Ramin did with his 1968 Barracuda. Here is a link if you have some time:
http://www.jefframin.org/barracuda.shtml
For now, and for the entertainment of some our members, where did you locate your rare fish? Is there a good story? What do you know about it so far?