I would definitely use atleast a 7/16 inch, .120 wall pushrod with your spring pressure and rpms. I have had excellent results using Manton. They have great customer service and will not steer you wrong. I have a 434 sbf with over .800 lift that I spin 8500 with 400 lbs on the seat and I have Manton 1/2 inch, .168 wall dual taper pushrods in it. I have never had an issue with a Manton pushrod. Put the biggest pushrod in there that you can get.
The pushrod to cylinder head clearance on my engine is roughly .0035. Don't worry about .520, that is way more than plenty. Tight pushrod clearance can actually help in stabilizing push rod deflection. As long as its not to tight. Just make damn sure that the push rod is not binding against the cylinder head when you rotate the engine.
You should be fine with a 7/16 push rod as long as the push rod doesn't bind against the cylinder head.(very important!) If you have aluminum heads it is not uncommon to have to clearence them a little.
Like others said, lightly touching will not hurt. After you get your final measurement you can have one intake and one exhaust made to check clearance and final length. I adjusted the length slightly after doing this, it also gave me a spare pair just in case.