I usually order from Forum Venders. But I have to admit to ordering parts from time to time from LMC. The reason is, that since I use their online and
hard copy catalog to answer questions I have that are similar to yours, I'd feel guilty if I didn't send them a buck or two from time-to-time.
I just pull up the site and check the part numbers. So, if you look at this page, for example, you'll see the "sameness" and "differences" in fenders...
...across the years and between GMC and Chevy.
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=23
My PIA has been more in identifying the *actual part*. Many times, especially on CL or E-Slay, the sellers REALLY don't know the year of the part
they are selling. I wish I had a dime for every part stated to be "67-72" when it was really a 69-72.
I'm sure you have all seen the same thing.