Sadly, I am somewhat of an expert on Tri Five sheet metal parts having purchased complete floor, both A pillar repair kits, a left front fender, rocker panels, inner and outer wheelhousing, driver's door, rear quarter panel, front and rear bumpers, inner front fenders, grill and tail pan - all for one car. My roof is original though. Note to self: never buy another Cleveland car.
Most after market parts do not fit correctly without rework or have defects. When I buy a piece of sheet metal from a retailer, with few exceptions, the part comes from Golden Star in Texas. Golden Star doesn't manufacture parts, the parts are manufactured from a dozen or more suppliers in Taiwan. For example Golden Star sells Dynacorn parts made by He Qing Industry Co. Ltd. Parts arrive via container ship, then are shipped to Golden Star which in turn supplies the retailer which in turn supplies the Mustang crowd.
The Tri Five segment is small compared to other markets. I don't know what parts He Qing Industry makes for Tri Fives but I am sure they manufacture some. Even if He Qing Industry knew about a defect they have no reason of fix it. Golden Star is small potatoes to them. They manufacture hundreds of parts for Holley, CJ Pony and others. In those rare cases where an engineering change is made or a defect is fixed, it can be a year or more before another order is placed and then it can be months enough product can be made to fill a container. Worse, if the defect is caused by worn out tooling the part will never be fixed. The time and expense of replacing worn dies for a low volume part makes no sense since there is no chance of ever recouping the cost.
The reality is that on the sheet metal side, it's a take it or leave it proposition which ultimately translates to the only after market part available even with defects is better than nothing.
Although Golden Star has really nice printed catalog and functioning website now, they only sell parts not expertise. If you have a problem, you are on your own. I choose to purchase from a retailer that is upfront and honest about the parts they sell. Before I ordered a door from my retailer, I asked what was wrong with it. They gave me the bad news straight up. That way, I can make an informed decision whether it's worth saving the old door or doing a ton of rework on the new door or scouring the country for a NOS or salvage yard part.