As someone who has seen his 305 under 9 ft. of salty water. I'm hear to tell you you can bring them back yo life. You'll probaly need to have them sep up a the next oversize pistons just so you can get good skin. Kanter and EGGE both have .20, .40, and .60 oversie pistons. A complete rebuild kit for the 405 will run you $1100. Been there- done that.
IF not a member, join Jolly Goodfellows 60-66 GMC Club, we have a national website and a yahoo BB. Many members have spare parts laying around and will be willing to help. I too love the V-6 as you can see by my avitar and links.
http://www.6066gmcguy.org/
http://autos.groups.yahoo.com/group/6066_GMC_Truck/