To Heli Coil or not to Heli Coil

[smsdm02]

Well folks, here is the problem:

I recently pruchased my very first truck (ever!); 72 Chevy Longbed, 350, 350 Turbo Trans. Runs great, drove it home about 150 miles, no problems. Got it home, drove around for a few days, still no problems. About a week after purchasing it, I noticed a small starter grind on the beginning of each key turn. Finally, about 4 days ago, it wouldn't start and I got a motor spinning sound with just a bit of grinding, no engine turn over.

No problem, I just check the flywheel for bad teeth (no bad teeth present), replaced the starter and good to go. It starts right up with no problems and I get it home, turn it off and go to bed. Get up the next morning and same motor spinning sound with just a bit of grinding with no engine turn over.

I pull the starter off (to test it, it's fine) and see that the hole on the edge of the block where the outboard starter bolt bolts in has a crack on the outside. So here's my question:

Do I try to heli-core the block where the bolt goes, or do I replace the block? I have access to another block, tools, engine lift, etc. (My Fiancee's dad builds drag trucks for fun)

Is it worth it to try, or will I just make it worse by trying to heli-core it?

Please bless me with your knowledge and opinions, O Gurus of the 67-72. Thanks a bunch!