Header install

How hard is it to remove the stock header and replace it with an aftermarket one for a 2.4L?

have 2 people makes life easier... since you are confined to the area near the firewall . So someone to hold it while somone else bolts her on... thats about it.

do you have to use any kind of silicone or something for the gasket to stick better to the block?

No, just make sure you use a metal gasket from a store like autozone or something, DO NOT use the cheapie cardboard/paper-like ones that come with the header. I used one of them and it blew out, I had to buy a new gasket, scrape off the old gasket off the header and the block, which wasnt fun, and take off the header again, and then put it back on again. So yeah, buy a better gasket lol

Edit - The heat will secure the gasket to the header and block well enough.