Are the factory MOPAR pushrods fine to use on a 440 with a hydraulic cam with 0.475/.494" lift?

Or is upgrading to soemething like these cheap insurance?
http://www.summitracing.com/parts/CCA-7840-16/

Lastly I assume with hydraulic lifter the 9.135" pushrods can take up a little slack if for instance different head gasket thicknesses are used or possibly even if heads/block are mildly milled?????

Usually the stock rockers are the weak point when using larger cams with higher spring pressures. The pushrods will often push through the seat in the stamped rocker arms.
It is best to measure for the required pushrod length to optimize the lifter pre-load, but the hydraulic lifters will work with a fairly wide range of pre-load, so if the milling/gasket thickness changes are not than around 0.030" then the stock pushrod length should be OK.

I'd mock it up & see where you're at & go from there & post how much preload you get