The procedure isn't something that can be practically typed into a forum response. Get your hands on a factory shop manual. There are critical steps like the sequence of loosening the cam towers. Get it wrong and you'll unwittingly have a DOHC engine.

I'm not aware of a longer oil filter. You're limited by the size of the oil filter housing.

What other work you should do depends on what work you're doing in the first place and why you feel you need to do that work. For example, you talk about the transmission slipping. Why are you pulling the head if the problem is with the transmission?

Sixto
87 300D