Fender reference points

[mechanixman]

Hello all,
So I've decided to start working on the front passenger side fender. My problem is I have two fenders that both need a substantial amount of work, so I'm thinking the best solution is to cut each fender in half, and use the front from the original, and the rear from the one I bought at a scrapyard.
My problem is I don't know where to measure to make sure when I weld the two new pieces together, to make sure the fender isn't crooked or an inch longer than it should be.
Here is a picture of both suspect fenders.