The 51516 is just a longer version of the 51348; correct. Spec's (other than physical size) are similar/same for burst, efficiency, flow, gasket, etc. Your knowledgeable source at your Napa is wrong; there is no difference in "valves" between those two filters. They both have a silicone ADBV, and also have the same bypass pressure rating, as of this morning.
You can most certainly use the larger filter. You won't gain any tangible measureable wear reduction, but if you sleep better at night (or it's beneficial by commonizing garage stock) then by all means, go for it.
BTW - Wix and other companies will not generally warrant filters used in "non-stock" applications. While warranty of your 10 year old car is not in play, the Wix coverage might be. You can check with them, but I'll be a spoiler and tell you they will not support the off-standard practice. So you're on your own in that regard.
Side topic, but directly related this this thread ...
Wix is constantly messing with their data on their site. These two filters (1348 and 1356) have been the easy swap for years. And a few years ago, there was clear reason to "upgrade" to the larger filter because the 1356 had a "better" beta ratio. The old "nominal" ratings were the same. But the "beta" data has been whitewashed on many filters to simply read "2/20 = 6/20" on more than 1/2 their catalog listings, and it's ever creeping in. But this morning, I noticed that the 1348 now has a "nominal" rating of 15um, where the 1356 has a nominal of 21um. First of all, the "nominal" ratings are pretty much meaningless, and the beta data is better. But when they "whitewash" the beta data across the board, it becomes meaningless too.
So, according to their site, as of today, the two filters have the same flow (7-9gpm), the same burst (275psi), the same bypass delta (8-11psi), and same gasket. Only the nominal is different, and it actually favors the smaller filter .... Go figure.
Also makes one ponder if you couldn't actually use Wix data against them. Think about this for a moment. The two filters are rated the same for all pertinenet info (nominal is worthless when you understand it). So, why could I not pick ANY Wix filter that would be the "same" as the OEM application? After all, if Wix own website states that the 1356 and 1348 have the same burst, bypass and efficiency, then what is the real tangible difference? How could they state that a 51348 is the ONLY acceptable filter when the 51356 is listed with the exact same performance criteria? How could they deny a warranty claim that the 51516 is "invalid" for this engine (or my Fusion 2.5L that also takes the 51348) when the 51516 is identical (according to their own data) except for length? Discounting any physical contact that would damage the filter, the two filters are IDENTICAL according to their own data. Therefore EITHER should be acceptable! How can they state that the 275psi burst rating of the 51348 is OK, but the same 275 burst rating of the 51356 is not? How could they rationally state that the 8-11psi delta P is correct for the 51348, but the 8-11psi delta P for the 51356 is not? How can they claim the beta 6/20 for the 51348 is superior to the same 6/20 rating for the 51516?
IMO, they are actually encouraging people to experiement, because they are publically taking away any differentiation of their products. If it fits, and has the same ratings, then they would be equal, if physical barriers are not an issue.
Food for thought ...