I am going to differ with the flow here ..........I like the ideas previously stated ,but here is mine .

Use that 305 ,its not a drag race engine by any means ,but will do just fine for your needs and you already have it purchased and know the history,the trucks been sitting ,so a having it running while you do the engine does not matter.If it can sit ,then that just makes using the 305 ,even that much better!
Even if it needs machine work ,like most any used block does ,then the cubic inches go up and thats a good thing... ,with having to purchase the 350 engine ,opposed to having a free complete 305 engine that is application specific ,meaning every piece will hook back up again ,no new connectors ,no wire harness to make or buy ,etc.......
Here is a video of my 305 with the xe262h cam .
I am exspecting a honest 275-300 hp at the engine.

http://www.youtube.com/watch?v=ld4vfbizOBI