It is not difficult to use the Mark 25 powerhead on a H tower. The problem might be finding an H tower and gear case in good condition for the $$ in your budget ❓ Not familiar with the details of the mate but a special spacer might be needed between the powerhead and lower unit ❓ The link you note shows a conversion gear case on an H tower and that was never standard for the 20H, it was available for the conversion to the 20H with a special tuned tower, powerhead exhaust adapter and gear case with 16:21 gears.
There is no difference between the 20H lower and the H lower.
A little history: Mercury developed the speed lower unit and gear case they called it the Quicksilver or "Quicki", it had 1:1 gears and was stamped with a "Q" on the lower unit placard. It was made to fit the KG4, KF7, KG7, KF9 and KG9 engines and a Q would be stamped on the placard like KG7Q to designate it. Mercury then developed a shorter mid section (tower) than the Q tower to further lower engine center of gravity and allow the engine to be raised higher and not have really tall transoms. They called it the 20H with a special powerhead different than the Mark 20 service engine. The H was an abbreviation for "HydroShort", it also had 1:1 gears like the Q lower unit gear case.
More discussion here:
http://johnsoldmercurysite.com/phpBB3/v … f=7&t=1413
Here is a Mercury add for the Quicksilver "Q" lower unit, note they say "Quiet Underwater Exhaust" 😆 , that defeats speed proposed and they were never raced that low bad for speed:
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