Replacing the air conditioning unit with a bypass pulley. where do i start?

That's not the way you do it. You fit a Bypass Pulley. It keeps the same belt, route and tension.

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Buy a AC bypass pulley, it saves a lot of hassle and is a bolt on accessory.

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You can also remove the a/c compressor and install a bypass pulley in its place. You can still use the standard belt this way, just run it to the bypass pulley instead of the a/c compressor. RockAuto has them at the link.
http://www.rockauto.com/en/catalog/chevrolet,2001,impala,3.8l+v6,1384084,heat+&+air+conditioning,a/c+compressor+bypass+pulley,10329

did you remove the air compressor ,and if you did you need to put something in its place for the belt tensioner will not hold tension . it will be easier to just replace the air condition pump or best known as the air compressor with a used one . this is my link to my garagehttps://www.facebook.com/TedsRepairs

They make an AC bypass pulley that you can put in place of the AC compressor.

In some cases this can be done-but you do not mention the engine you have-also, there are bypass pullleys that some sellers have for just such circumstances, google bypass pulleys for your engine-you sometimes can do this yourself if you keep in mind the rotation of all pulleys has to remain the same,

Yes they make an replacement pulley that take place of the a/c.You just will not have a/c.

It's the A/C clutch pulley. There is a special tool required to remove and replace it. You may want to check around for prices on the whole unit. you may spend more for the clutch and tool than the compressor.

You need to pull out the timing cover. 4-6 hours of work. Here is the step to remove the Cover, then you need to remove the Sprockets.

1. Remove the oil pan.
2. Remove the power steering belt if so equipped.
3. Remove the air conditioning belt, compressor and bracket if so equipped.
4. Remove the fan clutch (fluid coupling), with the fan and water pump pulley.
5. Matchmark and remove the crankshaft pulley(s). This bolt will be very tight. Use the counter-holding bar, properly installed, to hold the crankshaft while loosening the bolt. Use the pulley extractor or gear puller to remove the pulley from the shaft.
6. Remove the water bypass pipe.
7. Remove the fan belt adjusting bar. If necessary, remove the lower power steering bracket for access.
8. Disconnect the heater water outlet pipe.
9. Unbolt the timing chain cover assembly. Be careful to loosen only the correct bolts.

To install:

1. Install new gaskets over the dowels on the timing chain cover. Slide the cover assembly over the dowels and pump spline. Insert the retaining bolts and tighten them. Tighten the 8mm bolts to 9 ft. lbs. (13 Nm) and the 10mm bolts to 29 ft. lbs. (39 Nm).
2. Temporarily install the adjusting bar to the alternator. Install the adjusting bar to the chain cover and tighten the bolts to 9 ft. lbs. (13 Nm).
3. Install the heater water outlet pipe.
4. Install the water bypass pipe.
5. Install the crankshaft pulley and bolt. Use the counter-holding tool to hold the pulley and tighten the bolt to 116 ft. lbs. (157 Nm). If equipped with air conditioning, install the second crankshaft pulley.
6. Install the water pump pulley and fluid coupling with the fan.
7. Adjust the tension on the drive belts.
8. Install the air conditioning compressor bracket, compressor and belt if they were removed.
9. Install the power steering belt and adjust it.
10. Attach the oil pan.
11. Install the radiator.

No. There is a tensioner pulley which controls the tightness of a serpentine belt. In addition, the belt travels a specific path. Bypassing the air conditioning compressor not only requires a different size belt, it would require rerouting the belt and omitting one component would mess up the travel of the belt and may actually result in reversing the rotation of some of the components that the belt turns. In addition, the belt tensioner is designed specifically for that length and size of belt. You would most likely affect the belt tension, which would result in slippage or overtightness. Overtightness on the bearings of the devices the belt controls will cause that unit to sieze.