The owner of my old truck switched from the 36" Irok radials I bought to 39.5's.... as far as I know, he hasn't broken anything.  4.88's, Detroits front and rear, 30 spline Longs, duals (both 21 spline 2.28).

40's and 'yota stuff would last for me.  I'm very easy with the skinny pedal.  Comes from wheelin' locked Samurai axles with 33's, I guess.