New to the forum and still learning engine basics. Looking at buying an ls6 block but the owner said the crank got ruined from over tightening the mains. My main questions is, what is worst case scenario? Can the block be junk because of this? Should I proceed with caution? Can the main caps be replaced and I’ll be good? Any info appreciated.

 

If the guy is honest, he will let you take it to a shop and have it checked. Overtightened mains means that the mains went out of round...which isn’t easy to do on stock bolts, by the way. Can easily be fixed with a line hone...won’t need new caps.
Lots of other areas on an aluminum block to pay attention to. Aluminum is soft and if there ever been an issue, it easy to hurt it. Lifter bores, cam journals if it’s ever spun a cam bearing, cracked starter mount, hurt threads, etc.
I’d have a good shop give it a once-over and give you a cost breakdown on repair before purchasing. If he’s not willing to let you do this...maybe with a refundable deposit...I’d look elsewhere for a block.

 

he lives 4 and a half hours away so all that might be tough. The bores still have cross hatches but the rest you listed obviously a shop would have to check out.

 

Just like you said proceed with caution. If you were getting an LS6 or LS1 that was in sound condition I would say go for it. If that LS6 doesn't work out you can easily find a 5.3 ls truck motor out of a Suburban or Yukon to use in your project.