100 amp fuse keeps blowing

Long story short I replaced the engine in my truck with a 2000 same engine slightly different hook ups. Anyway I keep blowing my 100 amp fuse. I replaced it and can crank my truck. It runs fine. Put a meter on it with ignition on but engine off getting 12.56 VDC. Metered again with engine on and at one time it was 14.94 VDC I turned it off and moved and wiggled around some of the wiring harnesses then tapped on the voltage regulator and cranked it black up and now it is holding at 14.04 VDC even with my A/C on full. I put a 20 amp fuse between the negative post of my battery and the negative battery cable to see if it would blow but it did not blow so I tried a 10 amp and it did not blow. I can crank my truck and pull the positive battery cable off and the truck runs just fine even with A/C full blast. Nothing dims or drags down. But if I drive for a little while. I could get a month or sometimes just a few hours and poof fuse goes. What am I missing?