Can anyone point me in the right direction to solve my problem?

1982 Chevy K20 w/5.7L w/ 106K miles. 8 months ago the fuseable link under the hood went bad and I replaced it with a 30 amp in-line fuse. Worked great until last weekend. Went to use the truck, got nothing when I turned on the key. Found the 30 amp fuse blown. Replaced it drove to town, parked shopped, went to come home and the 30 amp fuse was blown again. That's the pattern when I need to go someplace, replace the fuse. The fuseable link is coming off the starter. When the engine is running and the fuse isn't blown, the volt gauge reads 12/13 volts. Only after the fuse blows does the gauge read 18 and the engine will still keep running until turning the key off. If you turn the key to the "on" position and leave it there without starting the engine the 30 amp in-line fuse will not blow. The fuse slowly heats up and blows while the engine is running and then the voltage indicator pegs to 18. Tried to trace out the problem with a Autozone wiring diagram but it did not help. The local Chevy dealer does not have diagrams going back to 1982. I thought a diagram of the charging circuit might help but I do not know where to get one. Anyone out there heard of how to fix this problem?