The easiest option would be to get a 3rd member from a '68-'72 C20, usually with a 350 and automatic, which has the 4.10 gear. That would be a direct bolt in. Pretty much any other rear would require modification to work with the trailing arm/coil spring rear suspension in the '61. If you were to use a later rear, it would not necessarily have to come from a deisel truck, gas engine trucks use the same rear. For something narower, you might look at vans instead of pickups. 3.73 is a common gear on the later trucks, you might be able to get a 3.55 gear if you look at Dodge Dana 60s.
