Apparently the AC condenser in my 76 Valiant is seized. Is it possible to rebuild the unit, or would I have to order a remanufactured or NOS one?

The condenser is in front of the radiator and has no moving parts to seize.

Perhaps you realy mean the compressor? Rebuilt units are readily available through any auto parts place or radiator/AC repair shop. I don't know which are good quality or not.
You won't know if your old one can be rebuilt or not until you know exactly the problem inside. The biggest problem I have encountered is great difficulty finding any repair parts for these compressors, but the rebuilders are getting parts from somewhere. (anyone know where?) Auto parts places no longer have any parts available for these units that I can find but they all seem to have rebuilts available.

The quickest and easiest will be purchasing a rebuilt unit

Condenser or compressor? Any part on your AC can be repaired or replaced with aftermarket parts...at least that's what my AC guy said.

Yeah, compressor.....that's it. Having a car with AC is kind of a new fangled technology to me lol. But the car's a 76 Valiant Brougham in excellent shape, it really doesn't need a whole lot of restoration. Aside from the AC, The wiper pivot gaskets need to be changed ( I have new ones on the way from Schumacher), and it needs a new carpet, the original one was removed by the previous owner. I was thinking about ordering one from JC Whitney, but I've heard horror stories about the quality of some of their products. Did these cars come with some sort of noise/heat insulators underneath the carpet?

Brougham models had extra sound deadening materials and the carpet was a deep shag, which really made them quiet. You can get a high quality reproduction carpet in the proper material and color from: www.accmats.com.

They are the suppliers for many of the restoration companies and I've bought from them several times. Their service has always been outstanding. They also have live person on line who will answer any questions. I've had them send color samples and have even sent paint samples for them to match colors.

For sound deadening under the carpet, I would suggest something like FatMat. I used that on my just completed truck and it makes a world of difference. It's similar to DynaMat, but less expensive.

If the A/C compressor is cast iron, black, and looks like a V-2 engine, just get a reman unit. If it is aluminum, round, and silver (could be black), get a new one, not a reman. The quality of those remans, is terrible, and I don't care who the rebuilder is.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225