Always replace the stock crankshaft bolt. ARP makes a nice replacement. https://arp-bolts.com/kits/arpkit-detail.php?RecordID=1367
Stock or after market harmonic balancers should be ok to reuse but inspect the rubber ring between the inner hub and the outer ring for deterioration. If it’s is cracked than replace the balancer.
To replace the seal correctly you should use an alignment tool like this one https://store.custombuiltmotors.com/tools-/cbm-motorsports-ls1-ls2-ls3-ls7-lsa-ls9-front-and-rear-cover-billet-alignment-tool-set-with-free-ls-.html
The stock crankshaft bolt is a torque to yield fastener so you will need a torque angle gauge (cheap $) for final torque. I believe the ARP bolt just requires a torque to Ftlbs.