pipe sizing?

does anyone have any advice on how to size header pipe sizes and exhaust pipe sizes and mufflers.  

Stick to standard sizes — don't go big.  

What I meant is what would be best for my 350
1&3/4", 2", 2&1/4", 2&1/2"primarys 3" collector? How can
I determine the best sizes from header to tip?  

2-inch or 2-1/2 will work fine without knowing your cam, carb etc. Having to guess what your 350 is in puts this all in general terms. Some trucks are easier to plumb than others for exhaust. The larger the diameter of pipe the more room is needed to make bends. 3-inch pipe is for Diesels or guys that want a drone induced headache.  

Thanks John, It's a 77 custom deluxe 20, 3/4 ton. The cam, carb and everything is stock. I want to start modifying it, and the exhaust system seems to be a good choice to start with, but I'm trying to do my research first. This is my first project vehicle and I want to do it right.  

How about smog concerns in Ca. It's a 77 the manifold has a small pipe that runs back up to the air cleaner, but I took that off and swapped it for a open air filter and chrome cover.  

Chad,

Sorry I should have looked at your profile, and I would have found out you are in Calif. To stay smog-legal you   will have to have your original stock air-cleaner. My '79 C-10 is smog-legal with a K&N filter, a Performance Distributor DUI and a Mangnaflow high-performance catalytic converter and muffler. Off the top of my head I can't remember which issue but we covered the build in 2007 after I ran 5,200 miles in one-week during Americruise. Any truck 1975 and below is smog-exempt in California.  

Does that mean headers would be a bad idea?  

People seem to think headers are some pipes, connected to the side of the head. That is a very bad assumtion. Companys like hooker, headman, clifford, even flowtech, have been studying and expearmenting with headers for decades! Its all about flow. What about scaveging. correctly engineered headers will actully creat small vacume pockets in the exahust stream to help suck exhaust gasses out. These people have spent millions in research and development. Thousands of hours on flow benches and test tracks. I would recomend you talk to the header manafacture of your choice and go with there recomendation. stock mainfolds will out perform a badly designed header.