1966 Mustang rear fender wells

Yes, the tab should be there. It's the cap for the rear end of the rocker.

It's not unusual for repro wheelhouse parts to not line up, force it.

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you can try to use 2" self tap-er to suck it up or big vice-grip or two/3 then make spot weld

I ran into that problem many years ago on a coupe. If you do force them together which is probably the only solution it could change the shape of your quarter. Make sure that you are satisfied with the quarter contour before you weld.

you're going to have to "mock up" the parts and compromise with your adjustments. There's no magic bullet with replacement parts.... even the most expensive ones require some fitting, although it's better to build things from the inside out, fitting your outer quarter last... if you still have that option. It's better if you draw adjustments from more than one part, but keep the outer panel free of as much stress during this process as possible to minimize distortion. Pie-cuts, relief cuts and trimming are very common in replacing sheet metal components on an older unibody car regardless of the age, make or manufacturer of the replacement parts. my best advice is to do one side a t a time, using the opposite side or even another car for reference.

I usually use self drilling screws to get things in place without "locking' them with a welder first... that way if you have to borrow from one part you can undo things and make adjustments fast and easy.

Good luck!

KT.

Welcome to body work?

It has pretty much been said, you has to "convince" it to fit how you want it too. You only other choice is trying to find NOS OEM quaters or a donner car, both will likely be very expensive and still require a little "convincing" to work.