Distance from the prop

How far away should the rudders be from the prop?
Just clear by an inch or two or a better distance?
I plan on using 10" wide and 50" long two of them.
At about a 1/4 the distance from center to the prop tips on
my big prop 64". ?????????????

My rudder is probably 14" away from the prop and works fine my neighbors boat has a 64" prop and has two rudders about 12"away from the prop that are roughly 24" tall and 16" long and have been working fine for almost 30 years. I don't think the distance from the prop matters that much as long as you clear the prop. Maybe with an airfoil rudder it might make a difference

How close is to close?

I would give at least 3" for the prop flexing and any movement in the motor mounts. I'm sure you could go closer if your brave enough. But it's there a reason you need to go so close?

No just a lot of stuff hanging off the back of the boat

I've always thought the farther away from the transom the rudder is the more leverage it has on the hull to push it sideways. It's kinda like the front of my boat if i push on just the bow of the boat is harder to spin the front around then it is if i grab the end of my grass rake 3ft out because it give me leverage.

It's hard to say how close is too close...Your rigging geometry is the biggest factor and just how much the hull will flex as you move in and out of the water...
Aerokirk railed a dock in his winter video and it popped the one rudder out of it's top mount...Luckily it leaned out and away from the prop as it came loose but the jolt could have easily sent it the other direction initially for a big disaster...
So "how close is too close?" is all relative to how strong the rudders mounting points are, how the engine/prop will move INDEPENDENTLY from the cage/rudders...And how much hull flex will add to the potential problems of stuff colliding ....