Replacing Valve Seals W/O Removing Head

Hi,
I am going to be attempting to replace my valve seals without pulling the head off. I will be using a leak-down tester to supply pressure to the cylinder to keep the valves in place. I am wondering if anyone has done this before and if they have any tips and tricks before. Mainly I am wondering if it is possible to do this job without removing the timing cover?

Thank you in advance for your help!

 

Back in my pinto days people would push rope into the cylinder through the spark plug hole to hold the valves up when changing the valve guides I wouldn't trust the air pressure to not slowly leak out

 

It is definitely an uncomfortable thing to do but I have done it plenty of times on other vehicles. And if the pressure is leaking out, you've got bigger issues and might as well pull the motor haha.