I want to install the pulleys and drive belts on my 350 to the new March Pulley system, along with a new billet air compressor and power steer pump. This car has the factory AirCon can I change the compressor only and retain everything else?, what about gas i presume the old system uses Freeon, with the new compressor will the old system be anble to have the new gas?

Yes, the new compressor will work with the old system. You will need to use the same oil for the new compressor that is in the old one. If it is R12 Refrigerant than it will be mineral oil. If the old system was retrofitted to R134a, you need to use the same type of oil that was used for the retrofit. The new compressor probably came with oil for R134a and you will need to empty it.
Gary

Filip,
If you want to put in a new compressor and retrofit the system to R134A refrigerant then
I recommend you let someone that has the right tools(vacuum pump, R12 and R134A compatible gauges) to do this as this is what it takes, as a minimum:

1. Go to your local Auto supply store and buy the R12 to R134A retro fit kit. It will have all the Oil, service fittings and O-rings you need. I also recommend flushing the system out so buy a can of system flush.
2. If the R-12 vehicle air conditioning system is operational, run it at idle with the A/C blower on high speed for five (5) minutes to optimize the amount of oil in the compressor.
3. Recover all R-12 refrigerant from the vehicle's A/C system. Evacuate the A/C system for at least thirty (30) minutes to a vacuum of 29 in. Hg, you see the Oil will hold some refrigerant so that R-12 must be boiled out which is done when evacuating.
4. Remove the compressor from the vehicle.
5. Remove the existing R-12 receiver-drier or accumulator-drier from the vehicle and discard. Allow as much oil as possible to drain from the A/C hoses.
6. Flush the system, all hoses, condenser and evaporator. Do not flush the expansion valve, blow high pressure air through it to remove any oil residue.
7. Change all O-rings, 0-rings on the receiver-drier or accumulator-drier joints to approved HNBR 0-rings; check and replace other 0-rings that have been disturbed. O-Rings are in the kit.
8. Add the 10 or 11 Ozs of the Oil (which came in the kit), put a few Oz in compressor, receiver-drier and condenser.
9. Replace the receiver-drier or accumulator-drier with a new R134a compatible one.
10 Replace the compressor.
11. Change any seals/o'rings at the compressor ports.
12. Put on the R134A service fittings to prevent any refrigerant other than R134a from being used. The fittings will be in the retro fit kit.
13. Connect R134a service hoses and other equipment. Re-evacuate the system for thirty (30) minutes using the R134a gauges. Turn vacuum pump off and see if it holds a vacuum. Wait a few minutes, if it does not hold a vacuum then you have a leak somewhere. Pressurize the system and look for your leak, fix leak and repeat this step.
14. If no leaks, Charge the A/C system with R 134a. Generally, about 5% (by weight) less than the R-12 charge amount is required. Old R12 charge was 60 OZ, or 3 3/4 lbs. Leak check the system.
15. Put all R-134 labels on the compressor and fittings. This lets the next mechanic know whats been going on. It is also recommended that a high pressure cut out switch be installled on the high side(liquid line, between the compressor and expansion valve)
16. Do an OPs check.

I didnt even get into the POA valve. It will need to be adjusted to compensate for the different pressures R134A operates on.

So, if your ready to do this by yourself, keep asking questions. i'll try to help out the best as possible.

Wow thanks for that. I think that this is something that I wont be attempting and will just let the professionals handle it, it is definetley a lot more complex than I thought it would be. Thanks for your advice