How to prune GIANT step over style apple trees!

Yes as @noisette47 has said, what you are aiming for is renovation pruning. Sacrificing one seasons fruit for a better structure and ultimately more manageable trees will be worth it.

 

Well I would tackle it one of 2 ways:
1.Do more or less what @noisette47 has said but I would be more drastic, thinning out about half of the shoots on the bushy bits then in the summer look up 'summer pruning of espalier apple trees' and shorten the new growth as they describe. You can make x shaped supports from sturdy wood to support the branches.
2. Cut through the horizontal branches 3ft from the trunk and let the tree return to a more upright shape then you won't need to support it. It will never grow that big as it would have been on a dwarfing rootstock. You would have to remove branches to shape it as it grows but can take your time.

 

Thinking about it, it looks like there were once more horizontal arms but only the top ones are left. I have found myself that the upper arms tend to shade the lower ones and the sap tends to go up so the lower branches are less vigorous.