Whoever owned this car before at some point has put some money into it engine part wise.
While getting ready to check valve adjustments today When I started to turn the motor over with the plugs pulled and a socket on the crank bolt I found it loose. Not just loose, but either stripped or broke as it wouldn't back out either.
I didn't mess around with it much more than that as I am not the sure the best solution. I'm assuming I could Helicoil it.
Thought about trying to get the bolt out if possible and run a thread chaser tap through it as I have a good bolt.
But Im just a bit lost on the best fix other than just swapping the crank out.

 

Yes a Helicoil is totally acceptable. Once you manage to get the old bolt out of course. Hold it with a wrench or something, drill and tap it for a 5/16" bolt or something, pull it out.

 

Easiest way to strip balancer bolt threads is to use it to push a balancer onto the crank stub which is probably what happened.

Multiple ways to remove a stripped bolt but none of them are easy.

Repairing it however there are only a couple of options. Depending how damaged the threads are, a helicoil is an acceptable repair. SBC engines use a 7/16 bolt. If the threads are in really poor condition, you can drill and tap it for a 1/2" BBC crank bolt.

The proper way to install a balancer is with a balancer install tool but if you don't have one of those, threaded rod, washers and nuts work just as well. Install threaded rod into crank. Use a couple of washers up against the balancer/balancer washer. Use a nut on the threaded rod and tighten the nut to push the balancer on. That way you're not twisting on the threads in the end of the crank. I know your balancer is already installed but someone probably just tried using the crank bolt to push the balancer on which stripped the threads.

 

If the original owner is my kind of dumb he may have used the harmonic balancer puller without the cone tip and mashed the first thread onto the second thread inside the crank. And then just rammed the crank bolt in.

 

Id bet you will be able to spin the broken piece out with a pick.

I put an ATI on my engine last fall and OMG what a nightmare. The interference fit is so tight and I was terrified id break the installation tool. the tool worked, you just need to keep rotating in while you tighten the nut down. The old truck was to put the camera in the oven fir a while to expand it first, might be worth a try unless ATI says no

 

What I've done on highway diesel trucks is to just heat the hub of the balancer with a torch and not the outer ring. Can't remember the temperature. Something like 300 degrees then quickly slide the balancer onto the crank. You can also heat it up on a hotplate. That minimal amount of heat won't hurt the balancer or crank seal.