It's been a while, so I barely remember. Basically, your rank is a value that's separate from your score/perfects/everything song related. I remember that, as a reference, I used the score of my songs (let's say I wanted to find the value for Full Chain, then I would search for a song which I got a Full Chain, then take the score, i.e: 4556812. Then I would search for that value on advanced mode, and once I find it, I would locate the variable (can't remember the name, but it's got to be something like chain or max_chain). Since by the time I decided to modify the values I already got the other trophies, I just replaced all the other values (for any score) with the value for Full Chain. I didn't even played all the songs in Proud, just the required 50 so I would have a rank to replace.
If you want to do more of a specific trophy, then you will probably have to go through all your scores one by one and change the value of the variable corresponding to Full Chain to get each difficulty separately.
EDIT: So, after digging for a bit, I found this. This is how the records of a song you're played look like. I don't exactly remember which ones corresponds to Full Chain, but I think it's maxChain. I highlighted the values those have my file, you can try playRank or fullChainClear and chrcwhat thye modify (you can see your changes in the song selection screen)