This is where things can get challenging, you never quite know why a previous owner did what they did ( or didn't do ).
I try to be really methodical when faced with problems like this. One thing at a time, try to determine a solid starting reference and work from there. Run through how each piece will fit and operate in relation to the other, make sure they can work together. And most of all, don't modify anything by removal means (drilling, grinding, etc) unless you are really certain it needs to be done. Shimming usually pretty easy, but sticking metal back on once it's a pile of chips really hard!
Good luck, you can figure it out. Just takes a bit of thinking on it.
-frank