Big block with manual transmission recommendations

The 4l60 is******compared to the 4l80, In fact the 4l80 is very much the same as the 3 speed one you referred to. I have a 2000 k3500 454 with the 4l80 and I just put it in 3 when towing.

Chevy does have a very good manual tranny in that era as well but I do not see them often and they are usually on work trucks not the plush leather crew cab like mine.

The 4l80 is the best rated tranny in the pre 2000's for automatic in a gasser. (probably diesel as well)
Good luck on your decision,
Paul