Don't want to loose google ranking (WP Super Cache)?

I am rebuilding a website, this old website has a specific html page which obtains great result for a specific 2 words search with Google.
I thought about using Wp Super Cache to lose as much as possible in terms of ranking (keeping a html page, same page address, aproximately same content).
Am I doing the right thing here?

If yes, can anyone tell me what writing mode I’d rather use in Wp Super Cache? Mod_Rewrite seams to be the best but is the php will have same effect on search engines? I just want to ensure that they see html pages, not php ones.