The NP435 is a great truck transmission, and I've daily-driven one in my first truck (73 Dodge W100) and second truck (77 F150) many tens if not hundreds of thousands of miles. That said, it shifts slow and for all practical purposes, is a 3-speed transmission with a granny 1st gear as noted previously. First isn't synchronized, nor is reverse, so shifting in and even out of those gears can be a bit difficult, somewhat depending on the condition of the transmission and how quickly you're trying to shift it. Otherwise, all of the standard Ford small block/Windsor type transmission options should be open to you given that the 300 I6 uses the same bellhousing pattern.
Since you said you would prefer an auto, the EB C4 would be a natural choice since it will work with the D20 directly and won't require a bunch of extra work or cost associated with many other auto trans swaps due to the adapters, cross-members, driveshaft interference issues, etc. Otherwise, AOD(E) is another swap that has become less popular over the years versus the 4R70W, which obsoleted the AOD-E and requires the purchase of a controller. If you're looking to drop that kind of money, for a lot more, you could consider one of the modern modular 6R80 or 10R80 transmission options given the relatively low-RPM torque curve of the 300, as both of these transmissions offer much narrower gaps between gears and would allow for lower axle gears and allow the engine to stay in a relatively narrow RPM band.
If you search, there have been some pretty detailed conversations about the modern automatic transmission swap options. Here's one from all the way back in
2017, but there are several newer threads included a the bottom of the post under "Similar Threads".