Sorry to be the one to tell you, but I presume that you are moving from a shore abode to a boat ?
If so, a lot of stuff will have to go !
Its amazing how much "stuff" we accumulate and I believe that there is a direct correlation between how much space we have and how much we allow ourselves to possess.
We have no spare space in our engine 'ole (trad) and we keep all our outside bits like water hose, pots of paint, spare ropes, tools, anchor chain, solid fuel etc in the bow locker and under the seats in the cratch (under lock and key).
The good thing is you won't need that patio furniture, pasting table, ladders, etc. from the shed, anymore !!!
Widebeam boats have a bit of room over the swims, but I have never thought they were that roomy.
Most boats that I have seen just keep some spare lube oil for the stern gland, the odd spanner, a dipstick for the fuel tank and a cloth in there.
I would guess that the main concern is ensuring that nothing can vibrate loose from its place and make contact with drive belts or prop shaft while in motion.
Good luck with the new build.
Best Regards
Mark
P.S isn't Tarifa a point in the Straits of Gibraltar ?