I did this on my 98 and it was actually far easier than I've heard.
I didn't need to jack up the motor or anything. I was able to just sneak them off without anything in my way
Take your time be patient getting them out
There are a few things to remove or just loosen up
To remove them
And putting them back is a trick
Because of the seal gasket
It falls out or it snags on other parts
I put just a dab of silicone on corners
And about in the middle of valve covers
Let it sit for a few hrs to secure to cover then install
My 2003 Mach 1 was a bear to change the driver side valve cover gasket! I think all 99-04 models are harder to clear.
I had to lower the engine enough so that the valve cover would clear the hydroboost.
You can either remove the bolt from the driver side engine mount and jack up engine or I used my lift and lowered the K-member.
Good luck!
Ryan