L26 crank is same as L28 as far as I'm aware it's just the bore is larger in an L28. So using L26 block might be to retain matching numbers?
In any case you've mis-read the Lewis Engines site, they are saying they build L26,l28 may as well include L24 also.
The L28 heads have bigger valves, that is their advantage and the fact that most of them are designed to tolerate unleaded fuel, earlier heads don't tolerate unleaded so well..