1. It may be front tiers out of balance. How far have you driven it? It could be flat spots in the tiers from setting in one spot for a long time. Other listed things are possible but I would do the tiers first. What year are the tiers? Older tiers are harder and will ride rougher than new ones.
2. May just be noisy. You can add some insulation under the doghouse and front floor to help dampen the noise.
3. Could be a bad donut gasket. When you can afford it I recommend headers better breathing and better hill climbing ability.
4. There are several posts on improving the airflow through the radiator on Chevy and Ford powered coaches. Basically you make a tunnel from the grill back to the radiator and block any air leaks so all the air goes through the radiator.
Yes the fan clutch could be a contributing factor.
5. My money is on the bad/ seized compressor. The good news is it is probley a GM product and there are lots of them out there get your part number. Hear is a sampling http://www.summitracing.com/search?keyword=Air%20Conditioning%20Compressors%20%20gm&dds=1
