So my pushrod tubes definitely have leaks. Is this job even reasonable to attempt with the engine in the car? Any pointers/tricks for someone who has never done this before?

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Awesome!! Thanks!!

A tip I learned while going through this a few weeks ago:
If the tubes don't come out easily- ie, your spending 1-2 hours trying to remove each one, stick a long socket down the tube , use vice grips to clamp the tube where the socket is and don't worry about trashing them. New tubes can be had for ~$15.

Clean well the ares where the seals rest and use a socket on an extension to tap the tubes in all the way. Engine oil is all you need on the seals.