The higher the mileage the bigger the risk. The rocker arm, push rod and lifter have been broken-in for a clean match. If you replace just the push rod, then the rocker and lifter are going to break-in the new push rod at the expense of excessive wear x 6 or 8 cyl.
Think of it like this. You've got a nice pair of shoes that you've worn to church for 8 years and suddenly you lost the LEFT shoe. You go to the shoe market and find a brand new LEFT shoe for display only. You convince the clerk that you'll buy just the one LEFT shoe. You put the new LEFT shoe on and the old RIGHT shoe on and walk to church Sunday morning.........................................?....................................?..............................???
------------------ "Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut
I replaced the head on my son's 84 Duke. Ended up with a bent push rod. Probably faulty install. Replaced all push rods and the motor purrrrrrrrsss. Like it should. Replacing push rods are a piece of cake. Don't cheap out.
During a top-end tear down, as recommended, I placed each used pushrod in a box/hole marked with cylinder and valve location. I was planning to replace the used pushrods back into the same location that they were removed from the engine. Unfortunately sometime afterward I accidently tipped the box holding the pushrods over, in which three of the pushrods came all the way out of box and holes they were stored in. I am unable to determine which of the three holes those three pushrods actually came from. Therefore I am not sure if it would be worse to use the wrong used pushrod in the wrong lifter/rocker location, or would it maybe better to replace with new pushrods. I realize using new pushrods would still not be ideal, as the lifters and rockers have an existing wear pattern. But I would rather not have to spend money on new: lifters, rockers & pushrods to resolve this problem if I can help it.
Go with the used ones .You have a 33% chance of getting them all in the right place and I dont think it is going to really matter any way .You have to set the valve lash before you fire it up and then I would reset it again after a thousand miles if it sounds noisy . The things that you do not want to mix up are the lifters .They wear in to the cam lobes and getting them wrong will cause problems .