Jorge, that will work, but the drawback is still the 20" transom gives you little freeboard at the transom at sea. Raising a 25" motor with a jack plate gets the motor up off the water, but still allows water to come in over the transom in a following sea at anchor.
On my 20' Seafari, I have a 20" shaft motor. At anchor here off the coast, the current, wind, and seas rarely align. Often the current and the seas are at nearly 180* to each other. That means I'm constantly taking water over the transom while at anchor. The water runs in the rubber boots for my control cables and steering cable as they are below the top of the transom in the engine well. Which means my bilge pump spends more time pumping than not. Even with good pumps and batteries that makes me nervous.
One of my long-range plans for my Seafari is to close in the transom, put a Hermco bracket on it, and repower. I'm waiting for MEG's Vision 3.0L diesel (735 lbs)
. I'll put my current motor on the back of a skiff.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury