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I made a set of Stauffer style tuners from $8 Chinese mandolin tuners. If I do it again I will use a better set of tuners and not follow the original guitar in having the posts above the worm (the original tuners are made differently than modern tuners and have plates on both sides of the cogs and are held together with tiny cotter pins). With steel stringing they tended to pull the cogs away from the worms. That caused the cogs to wear prematurely. I knew it was not a good practice, but thought I might get away with it. When I changed out the cogs and strung it with nylon strings it worked O.K. mechanically, but the sassafras top didn't lend itself to nylon strings (it sounds good with steel strings)
So I transplanted the neck onto a new body, and made a "crossover" guitar.

Could you go into detail of how you made the posts longer? I seem to remember you soldered a sleeve together?

On the original it doesn't seem like you would be able to have the machine cogs on the downside because they won't fit in the routing at the 6th string.

To lengthen the posts I used brass tubing. I popped the tuner buttons off the Chinese tuners and soldered tubing that had the inside diameter the same as the outside diameter as the steel shaft. I then had tubing that would slip inside the initial tubing to bring the size down to what the buttons required. The tubing came from a hobby store and was what I had on hand. it worked O.K. but required 3 sizes of tubing and looked messy. When (if) I do this again I will find some metal rod the same diameter as the tuner shaft and just use a small piece of tubing to connect them.
You are right - if a person makes an identical copy of the peghead the posts will be above the worms. I used the original as a pattern so that is how they ended up. This seems to work O.K. for the lower tension of gut strings (time will tell) but not steel. If the post was more rigidly held as it is in the original tuners by bearing plates on both sides of the cog it might also hold up O.K.. Again if a metal bushing was fitted around the tuner post in such a way as to press the cog into the worm to counteract the pull of the strings that might also work.
In a "do over" I would be inclined to modify the design slightly and have the worms above the cogs. The string tension on the posts then acts to keep the cogs meshed with the worms.
Here is a supplier of small tubing in various metals and thin metal sheets for making the back plate as well as all kinds of small useful metal bits:

https://www.ksmetals.com/

"To lengthen the posts I used brass tubing "

That is not what I meant to say. What I meant to say is - "to lengthen the shafts I used brass tubing". Actually I -shortened- the posts and redrilled the string holes in them lower down. There is less than 1/4 inch between the plate of the tuner and the face of the peghead. The posts originally stuck up too far and created some unwanted leverage.

As a side note for those that might be interested - 12 string guitar tuners use the same tuner spacing as mandolin tuners and may have the advantage of being 6 on a plate.