Raisedinal,
There is nothing wrong with a two bolt main block. They were used for years and years. Techinspector1 is right about the crank. Cast cranks will work just fine, unless you plan to turn in the 8- 10,000 RPM range. Then in that case the 400 would not be a good choice anyway because of the short rod. If you replace the heads, make sure that you drill (or have it done) the steam holes in the heads to match the block.
Ed