As for building a streetrod, you need to decide what type of actuation you are going to use. Often it is easier to go with a hydraulic slave cylinder setup rather than mechanical linkage.

To determine what to use, you need to have everything installed in the chassis, and have the body on the frame so you can figure out the floor and firewall issues at the same time. Will the pedals go through the floor of hang down under the dash? It's not just about what fork to use...
Most forks are stamped sheetmetal. Any one of them will do for initial mockup. If you want a forged fork, go with a 50's-60's truck, they are on Ebay all the time.

here's a quick search http://www.classicparts.com/searchprods.asp

Here's some ideas for a hydraulic setup
http://www.novak-adapt.com/catalog/clutch_components.htm

Doing a search onEbay for " clutch fork chev* " will give you some ideas as to what's available. You are most likely going to have to just pick one you think will work and start the mockup process. Worst case is you end up buying a second one once you have the whole setup figured out.

I.I. #3174