The belt rolls the A/C pulley just fine, but the compressor itself cannot be turned by hand? Pulley bearing is OK, compressor is seized. If this is the "R4"
Radial
4-cylinder style compressor--and it surely is, from my memory of that model year--they were underdesigned junk prone to failure; particularly the early years. They may have gotten better as the design aged. This coming from the company that built an absolutely bullet-proof A6 compressor (
Axial
6-cylinder) twenty or thirty years prior to the lightweight R4.
Rebuilding the compressor in-the-field is no longer recommended; I'm not sure it was EVER a good plan with the R4. A person can buy a rebuilt compressor having a reasonable warranty, at any parts store in North America.
Refrigerant work is regulated, purchase of R12 refrigerant requires a compliance certificate--which although remarkably easy to obtain, is still a pain in the ass.
http://www.ase.com/Tests/cfc.aspx
Which means that once you repair the system, you're probably going to have to take the car to a licensed, certified, (EPA-compliant) service center for recharging. If there is any refrigerant still in the system, you'll have to have an EPA-compliant shop reclaim the existing refrigerant before you work on the A/C system.
There is no logic in replacing the compressor on a vehicle that old without also replacing every o-ring in the system, (Do NOT use cheap-junk Chinese O-rings!) and the dessicant pack (which means replacing the accumulator or whatever part houses the dessicant pack.) It's a good idea to also install a fresh orifice tube and screen. I would not do this work without also having the evaporator and condenser flushed and tested by a local radiator shop. You will probably find that the evaporator box is packed half-full of leaves, twigs, and maybe a mouse nest.
Conversion of the system from R12 to 134a is common, but involves it's own complications, starting with requiring different high- and low-side service fittings also available at any parts store. Opinions on the suitability of the existing A/C rubber hoses when used with 134a are mixed. Some say the old hose is fine, others suggest having the rubber hose replaced with "Barrier Hose" specifically marketed as suitable for 134a.
Short story:
You better really want the A/C to work, 'cause this is not going to be cheap or easy.