The easiest manual transmissions are going to be the NV4500, NV3500, and SM465. They all came stock in these trucks and are basically plug-and-play to get driving. You can spend time cleaning up wiring harnesses and adjusting the tune to work better with a manual, but all of that is absolutely not required to go down the highway.
I put an NV4500 in my 98 5.7 K1500, best thing I ever did. But keep in mind it's a TRUCK transmission. Granny low 1st, wide spaced 2-4, 5th gear overdrive. The granny low is real nice for tight and steep trails, and the wide spacing hasn't given me any issues. For everyday driving it's basically a 4 speed, I hardly ever use 1st gear because it's so low.
For a closer spaced (car-like) tranny without granny low, an nv3500 will also work, you won't hurt it with a stock 5.7. 4x4 NV3500s are a lot easier to find than the NV4500s as well, took me about 2 years of casual searching to find my 240k mile 4x4 NV4500 transmission. I held out for the NV4500 because I wanted one that would hold up after grenading 3 4l60Es. Even after 240k miles in a different truck it still shifts smooth with original synchros.
I don't know anything about the SM465.
As others have said the 4l80e is the most popular one for guys wanting a stronger automatic transmission. They can handle a lot of power and a lot of abuse.