At how many RPM should I shift gears in a 1981 Foxbody Mustang?

Daily driving or quarter mile?

Higher speed requires more power, more power requires higher rpm's, and a 2.3l, 3.3l and 4.2l are all going to be different.

Daily driving you might find it comfortable to shift out of 1rst at 2000 to 2500 rpm's, but you may need 3000 to 3500 or highway speed before shifting out of 3rd.

Quarter mile pretty much at redline.

Some time ago a few cars came with little lights on the dash telling you when to shift for best fuel economy, pretty much everybody said it felt like you were short shifting.

High enough so it doesn't bog in the next gear when you upshift and low enough so it doesn't redline the engine after downshifting.

A more precise answer depends on what you're looking for...economy or speed and which engine is under the hood of that Mustang.

It all depends on if you want acceleration or fuel economy!

Ford engines tend to be able to take 10 grand with out a problem when new, but at some point Ford installed a rev limiter. So the idea is to shift right before the rev limiter kicks in or would kick in if the desire is to accelerate as fast as possible.

If the desire is for fuel economy then shift at about three grand like it was some cheap GM engine.

Always a half degree before redlining the RPM