Cannot speak about a Seagul, but most manufactures give a guide as to what shaft you need in relation to the fixing height off the engine above the waterline/bottom of the boat.
From my experience, the vast majority of Wayfarer owners opt for the short shaft. This is partially down to the weight of the engine and also the fact that many can then be housed in the rear buoyancy tank.
That being said, it depends upon whether you intend to fix it to the back of the transom with wooden “pads” to take the load, or whether you intend to use a removal bracket.
My boat has the latter which has the effect of raising the fixed point above where it would be on the transom, and so I opted for a Mercury 3.3 Long Shaft. This puts sufficient shaft below the waterline for good propulsion, steerage and economy.
Hope this helps