I spun some berrings in my 400 a few weeks ago and am putting in an old 350. I was planning on rebuilding the 400 this summer when I get some money, but I just got an offer for it. A guy at work wants to trade me a complete 350 (read rebuildible) for my 400 block and crank. He says it is a 509 block and is very rare. I don't know too much about the 350 because he is trying to find one right now. I am looking at a lot of money if I was to rebuild the 400 because some connecting rods are trashed, the crank needs to be ground, and the block line borred. That is just what is on top of a normal rebuild. My question is how much is a 400cid 509 block and crank worth? Should I demand more than a rebuildable 350 for it? Another thing is that I have been told that my 400 heads should be swapped out (if I was going to rebuild it), and I would have to buy some anyways (on account that they had small valves 1.74, and I want 1.94 or so). If this guy gets me a 350 what heads should I tell him I want, so that I can get 1.94 valves? The guy is an engine builder and I was thinking of asking him to build and instal the rotating assembly in the 350 he is offering. What would it cost normaly so I can compare to the 400's value?