combining inline tube prop valve and adjustable rear prop valve

You don't need the inline type prop valve if you are putting in an adjustable rear prop valve. You only need a distribution block (2-way splitter) up front; that is what I would do to simplify things.

Based on some reported experience of others here, with the 10" rear drums, you may not need any rear prop valve at all. So you might want to install and test to see if the F-R rear brake balance is inherently OK on a level gravel lot before installing any rear prop valve.