What specific parts should I upgrade for a 4L60E rebuild used for light towing?

Upgrade? I don't know what you mean by "upgrade". If you have damaged "hard parts", then they have to be replaced. The replacements with be the same parts that it came with.

Moving past the premise that my 4L60E transmission is currently in the repair shop due to a worn out 3/4th clutch. In order to slightly 'beef up' my tranny rebuild for weekend trailering a 2000lb boat in moderate terrain, what specific parts might I ask to be installed (vs factory OEM)? ie: 4 vs 5(? or more?)3/4th gear clutch pack assembly, corvette servo?, beefier sun shell, 5 pinion planetary?, bigger boost valve, single vs dbl gauge sprag, 7 vs 10 pump rotor vein, and I have read several forums talking about having the valve body drilled out in a certain passage? These are the possible upgrades I was hoping to get expert advice on. I would appreciate any suggestions that would help me decide during my rebuild process. Thanks! Tom

All I really suggest is beefing up the cooling system. Add an external trans oil cooler. And go with a higher ratio of coolant to water than 50/50, like 75/25. You live in Arizona, heat is your biggest enemy there.