Congrats on the cab work Mark! you did an awesome job on the rust removal and finally won the battle. You worked like a machine on that and make me feel lazy at times, but in a motivational way
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If I remember correctly GMC pauls carries Tri Plus/key parts body panels which you can buy in person at classic auto located at 89th and I35 (moore/OKC border) so no shipping there. i have a fender from there but I only test fitted it before tear down and all my panels where pretty off as far as alignment goes. Maybe call GMC pauls and Classic and ask them which brand steel parts they are using but I am pretty sure both sell Tri plus.
For inner fenders a friend at work bought his for a 72 from certi fit in okc, they are lower quality panels but he said they fit great and they are only like $59 each. He also bought his fenders there and they look good and line up pretty nice but they are thinner than tri plus versions.
also with these Oklahoma stores you can actually return stuff if you are unhappy with the fit.