One of the first things I did when I purchased a used 336 was put a receiver sight onto it. Lyman works, but the the Williams Firesight FP 94/36 is much better.
Williams Gunsight has a website that you can order from (google for it, as I don't have the link handy).
One thing that I would avoid is a tang sight. I tried on the 336, and if it wasn't for the polycarbonate lenses in my glasses (the "dink" in the lens really torqued off my optometrist too) I would have damaged an eye. I don't think the 336 has enough distance at the tang to suitably hold one.
Like I said, the Williams Firesight works the best.