The only thing I'd consider if rebuilding a 305 would be a slightly different cam, a different intake perhaps, and headers.
The 305 has some real problems when it comes to performance mods, and although it can be done, it can't be done nearly as cheaply as a 350 can.
More HP is easy, but to get it anywhere near what a "mild" 350 will do, would take more time, more money, and not have anywhere near the road manners the "same" 350 would have. I know people that have spent a fair amount of money and time on small CID GM V8's, but it's a lot of money for not nearly as much gain as someone see's with a 350+ CID motor.
Don't think rebuilding a motor makes it worth more, ESPECIALLY a 305. Buddy had a perfect running 1986 HO 305 (1986 Monte SS) that anyone that wanted to, could here run in the car, and had no takers for over a month, advertised in a national (but also large scale local) trader magazine. $300 OBO.
Besides, when dealing with a motor from Joe Schmoe, most people won't trust you as far as they can throw you. Ask anyone thats bought a couple of "good" trannies or engines how many of those were actually "good". The majority of people are honest, but if I'm going to spend $700+ on a "new" motor, I certainly wouldn't buy it from an individual I don't know, and have little, if any, legal recourse with if something goes wrong with it. $200, I might take my chances (and have) but anything over that, no way.
If you wanted to use it and a 305 suits you fine, sure, do an almost completely stock rebuild on it. But just look at the Chev 307's and their popularity to see what company the 305 is in.
