Had an issue late last year where the car just wouldn’t crank. Had power everywhere just no crank. Replaced the starter and ignition switch. Still nothing. Got good power everywhere just nothing when you turn the key to the start position. Crossing the starter makes the car crank. Walked away and come back to address problem further and now I’ve lost power at the fuse block. Any help would be much appreciated.
My 69 does that also when I first put the car together it fired right up , broke in the lifters still firing up perfectly. The next day nothing, finally figured the neutral safety switch was bad. Even today it will still not start unless I put it in neutral then push hard to park, then it fires right up. Eventually I may adjust the shift rod. You may also have a wire shorting out somewhere, which will take a lot of time to track down
The reason you have lost power at the fuse box is maybe when jumping the starter main terminal you might have crossed and hit ground and have now fried the fuse links . .