My '71 LT-1, to my understanding, was built to run on 91 octane, but that was back in the "leaded" days. It seems to run fine on today's 91 octane, but I was wondering if there is any longterm damage being done to the valves. If so, what can be done to reduce the chance of having major work to do down the road?

Your LT-1 was designed to run on something like 98-100 octane gas, but gasoline is rated and measured differently now. If your car runs OK on it, it's not really a worry.

The engine was also designed to use leaded fuel as you say. The lead acts as a valve lubricant, but also contributed to pollution. Since the lead was removed you should buy a lead substitute and add it to the gas. There are several brands available and are generally available through Pep Boys, Advance Auto, Auto Zone or places like that. Just follow the directions.

If you ever have the heads redone on the car, or ever replace them, it's routine to install hardened valve seats so unleaded gas causes no problems. Car manufacturers have been doing that since the lead came out of gas in the '70's.

I have heard that GM, knowing they were going to have to start de-compressing/de-leading engines in the near future, started putting the hardened seats in the heads in 1970. Don't know beyound a shadow of a doubt that this is true, but, one of the people I got it from is a VERY well respected, highly regarded man who is one of the ONLY people that GM has EVER let rummage thru their files.(He also happens to be a distant cousin of mine...with the same last name...)
That being said, I would also have to reccommend a lead additive on an early LT1. The octane rating isn't the issue anymore, as 'Slinger points out. It is indeed the effects of the valves slamming back down onto the seats with no "cushioning" that the lead provides, as well as the lubrication of the guides.

I have heard differing stories about the "preparedness", or lack thereof, of pre-'74 cars with respect to the eventuality of unleaded gas. I have even heard that late '71 Vettes may be more "prepared" than early '71s.