If you can not afford all the parts at this moment, save up. Meanwhile clean/check the parts you will be getting. This way if you want to paint/research etc etc, it gives you more time.
It is much easier to put in a slide the rear set of headers in with the head off. Plus, you'd end up redoing stuff 2-3 times if you do it in different rounds than all at once. To save money.. it's wiser to do it all at once. I believe I had one of the most expensive builds going. Each part I got was my last and I probably put on 15 different LIM gaskets etc lol.
While I find it impossible to believe your heads are leaking oil, Let's not talk about that or go there. Merely a point of mention.
