Nobody who sells a 50 yr old port wine is going to be making swill. You can pretty much decide what you want to spend, and pick up a bottle at that price, and have confidence you're giving a nice gift. That said, Taylor Fladgate is classic (do not confuse Taylor Fladgate for the similar-sounding "Taylor's" in the US--the former is Portuguese and is world-renowned (and commonly referred to as simply "Taylor" or "Taylor's" outside the US), the latter is $8/bottle NY stuff that's not what you're looking for). Kopke is also pretty classic. Both of those labels are pretty easy to find, though 50 yr bottles may not just be sitting on the shelf at your neighborhood liquor store. But like I say, any 50 yr-old port is going to be something special.
You're in Texas...do you have a Spec's nearby? If so, try visiting and asking for an appropriate gift for a 50th birthday, and see what they suggest. I'm in Houston and have access to their home base downtown, which is like the Home Depot of alcohol and staffed with really knowledgeable and helpful folks, but I've also gotten very good help from the staff in my local neighborhood Spec's too. Not sure if their staff at far-flung outposts are as consistently reliable, but it's worth a shot if you've got one close. I've also had good luck placing special orders at my local store and them transferring the product from wherever they had it to get it to me.