Anyone know anything about Nailheads? Specifically I need to know if all 401 balancers are interchangeable or if they are balanced differently in different years.

thanks  

several items from the 364,401, and 425 interchange. i am not 100% sure but the balancer from any 401 or 425,should work. the only difference in the two is the bore size (4.1875 vs. 4.3125).  

Yes they will  

It probably won't answer technical questions, but here's an older CC article on the history of the Nailhead that you may find interesting:

History of the Buick Nailhead  

go to  http://buicks.net/shop/reference/nailhead_casting_numbers_p.html and   http://www.clubhotrod.com/forums/showpost.php?=5ble4bbf739e16c0afd29b3e4cde45  i found this info very helpful as to what fits where etc  eg v6 buick neoprene rear seals400 455 buick front seals,  v6 cam bumper threaed type350 buick stud kits   400 455 buick rod bolts/nuts 400 455 buck rod beearings  400 455 buck valve locks   400 455 buick ring gear  57 to 66 only   avoid engine from 53   they suck   fyi the list of mistakes   #3  balencer bolt nut not tight enough   this allows the balancer to slip   which ruins crank/balancer  

Everything you need to know about Buicks is right here-V8Buick.com