If I remember right, 6498 means it's made of the alloy that has a little more tendency to blow up. 6061 is the new Luxfer alloy...6351 I think was the old one. 6351 has problems with SLC, sustained load cracking. They develop cracks more readily than other alloys of aluminum. Depending on the year of the original hydro, alot of places won't hydro them even--my local hydro place won't anymore, they stopped a month ago. If you have them VIP'd, they could tell you if there were any visible cracks. If there arne't, and someone willl hydro them, get it done. Make sure to have the LDS vip them again when they come back from hydro--sometimes the older tanks will develop small cracks after being hydroed. If they are doubles though, you might could sell the bands and the manifold on scubaboard or ebay for a little bit of money regardless. Then you have two big aluminum flower pots. Or, attach pipes and you have two good baseball bats for Paul Bunyan. Get them looked at by an LDS first though, there is a small chance you could use them. I say small because it seems most places are afraid of the older tanks and won't touch them regardless (that was the answer from two out of three LDS's) The only local shop that would vip mine basically told me "this vip is only good here. no one will fill this around here except us when they look at the hydro date. it sucks, yes, but people are scared of these tanks becuase some ofthem blew up."
[edit] while the last hydro is important, so is the first hydro. The rule of thumb locally seems to be, I believe, no tanks hydroed that are older than 88. Some places are rumored to only fill tanks on the original hydro, but those places are rare. Regardless, that 6498 kills you. no pun intended. those four numbers will scare away most people.