I am working on a 1978 Chevy 400sb and someone told me if I put heads off a 305 on this motor I will get 10 to 1 compression ratio..Does anyone know if this is true or not and are there only certain years that fit?? thanks  

What is the 400 bored too, what type of piston are you using flat top, dish top or dome.  

With the big bore of the 400, I don't think those 305 heas will work. The combustion chamber size is too small.  

I wouldn't recommend it 305 heads have a combustion chamber between 53 and 58cc's. Also 400 heads have more more water jacket openings for increased cooling that weren't used on smaller small blocks. In addition the bore on a 305 is 3 3/4" bore, the 400 has 4 1/8" bore; that means that the the combustion chamber would be about a 1/4" all the way around the inside of the bore on the 400 block.  

I build 350s with 305 heads but I know that putting them on a 400 just will not work out in the end. Even if you completely prep them for the 400 (steam holes) and use the biggest gasked or dished pistons, it will run but don't expect to go any were in a hurry. I do not beleave that the 305 heads can move any of the air that a 400 need and or put out. I have never done this and do not know anyone who has. I really beleave to that the only way to know the truth is to do it yourself and see what happens. Save a little money while you got it together and get all the parts that you know will work on it.  

I'm not familiar with the 400 either but I would assume those 305 heads if you can even get them to work? would be WAY to small to make any kind of real hp. you want atleast 64cc heads with atleast 200cc runners upto 220cc or so. If you go for a stock set of heads ATLEAST find a set of ported heads. But I would look on ebay etc for a set of worlds,darts or gm aluminums there on there now for real good prices. highcompression doesn't mean a fast motor you need heads that flow well to and there is usually ATLEAST 75 more hp with like a set of worlds than a set of stockers, So you atleast want a set of ported stockers.

Here is a head flow chart with hp#'s you can see why a good set of heads is important if you want a motor that can make power.  www.users.erols.com/srweiss/tablehdc.htm  

also you can see why you can't beat the new bowtie vortecs brandnew ready to go for 700$ if you want to spend 2000-3000k on a set of heads they'll make another 200 or so hp.