When to patch and when to replace?

[75sierraclassic]

The cab on my 75 is starting to get fairly rusty. The cab corners have big holes, both rockers need to be replaced along with the outside edges of the floor. I noticed that the front cab mounts are looking pretty thin and are on their way out. My problem is, should I start cutting and welding, or should I just junk it and find a rust free cab. When is the point to no return? I'm not sure what to do, I know a lot of you guys have dealt or are dealing with this. Whats your opinion?