what's a original 7 1/4 duster rear end worth

As stated most people give them away..
I wouldn't do free maybe like 40 or 50 dollars just so the scrap metal guy doesn't come pick it up. or if you're in the market for a good wooden back scratcher you could put it on barter on Craigslist... LOL..
reason being is because 90% of the people do exactly as you do they upgrade their rear end and upgrade to bigger Bolt pattern... I guess there's the occasional die hard slant six guys that look for this stuff but generally their slant sixes aren't breaking 7 and 1/4's...
You could always get lucky if you got time to just let it sit out there you might be able to get a little bit for it but it's not going to be people banging your door down for it and also there's the how long do you want to hang on to it end of the deal as well...