Due to the fact you're towing, something bigger than the 235 would be ideal. A 261 would probably be the easiest install. The 292 would require engine mounts and some fabrication. As far as transmissions, go sticking with the SM 420 should be fine. You could probably get away with a 3.42 or 3.55 rearend with the 4 speed also. The NV4500 from a newer truck would be a good upgrade, giving you overdrive.
As far a rearends go, The Dana 60 is a good option. There's plenty of them out there, and dozens of gear ratios.
I have a Full Float 14 bolt for my 3600 truck. They come in a couple different widths, the 67 and 63" versions. They had huge brake drums and shoes. If you can't find a 63 you can use your hubs (with corrected bearings) from the 3600 truck and adapt some brakes for use. GM had them in a number of gear ratios, 4.11 being the most common in the older trucks. 3.73 and 3.42 is a little more common in the newer trucks.
Another thing to consider is updating your brakes. There's a few guys out there that have successfully did their own disc brake conversions. You could use drum brakes from a 1 ton as an upgrade also.
