Most 400 blocks are the same ... for the most part. There are differences in motor mount bolt holes, but some blocks have them all drilled and tapped. But the short blocks are as different as the heads (from motor to motor). In fact, each motor/head combination also had a specific cam. If the same size motor had adifferent set of heads, it almost always had a different cam, too.
The cam in the '72 YX is pretty lame from a performance stand point. Bjorn was able to improve his '72 400 with better breathing heads and higher ratio rocker arms. The change in rocker arms adds lift at the valves. The biggest difference is the larger intake valves to add flow into the cylinders.
The best improvements in performance will come from matching the carb, intake, cam, heads and exhaust together - and at the same time - as Brett pointed out.
