From your intro thread, looks like you're talking about a '49-'51 Merc. I don't think you'll have any problems.
You can make them work, but they are a little different. 49/50 are the same, 51 attaches on the front edge differently, and has the extension on the back that's not like 49/50.. sorry for the smart*** remark, you could have been talking about anything
Yeah, stupid me was so engrossed with the subject matter I forgot to say which car I know 51s are different but I prefer that look Thought maybe you could just cut off the 50s and replace with 51s
OK you're talking about the upper quarter panels, not the lowers. I've read of people using 51 uppers on 49/50, but don't know if they replaced the whole piece behind the door, or pieced it together. I've seen NOS uppers for sale, it looks like a big job to replace the whole thing. 2 door upper includes the quarter window.
I've got cutoffs from a '51 parts car that might make things easier, and an extra '51 rear bumper I could sell. The ends of the lower quarter panels are different as well although you could graft those onto your '50 lowers to make it work. It's been done, and was even a popular swap back in the day, check out Winfield's '50 with '51 quarters, prior to the chop job. I guess he bought new panels from the Mercury dealer to do this.