Here are answers to your questions:
1) Yes, it is possible to swap valve springs while the head is still on the engine. You will need adapters to connect compressed air to the spark plug hole of the cylinder you're working on. That allows the valve to stay in place when you remove/replace the spring.
2) Here is a good and fairly brief description concerning setting preload. All preload is is the spring installed height.
Valve springs are probably the most stressed components of an engine. Yet they are also one of the most oft-overlooked parts in the entire engine. The fact of the matter …
www.onallcylinders.com
3) It is usually called out by the cam manufacturer. Assuming you're still running a stock cam, you should be able to just ask at your local parts store for valve springs for a 1969 Ford 302. If you have an aftermarket cam, you'll need to know the specifics for that cam - I.E. manufacturer, cam model, etc. Then you should be able to locate all of the information you'd need to purchase valve springs - open/closed pressures, installed height, etc.
4) You may need valve spring shims to adjust the installed height of the springs in order to meet manufacturer's specifications. Here is a link for Comp Cams shims, but there are probably as many manufacturers that supply shims as there are cam manufacturers - probably even more than that! Really all you need to search for is "valve spring shims" and a plethora of options come up on Summit's website.
I hope this all answers your questions!