I don't know about modifying the lower, but I think it is important to shoot a different upper, instead of the original M16 upper.
I intend to use my M16 lower with a Colt LE 10.5 inch upper.
A complete M16, with original upper and lower is worth about $5,000-8,000 more than an M16 with a non A1 or A2 upper. Do the search on gunbroker, and you will see a huge difference in pricing.
I am going to set aside the original upper and just keep it as a collectible.
Also, here is what a dealer told me. If you decide to mess with the lower, be very careful you do not lose the sear. He says that the most common way that sears are destroyed is that they are lost. You're out in the woods, or some other setting, you open the gun, and the sear falls out due to some faulty reassembly, etc, and your sear is gone, and then, for whatever reason, you can't find it or don't know that it's gone. He says that he fields calls from people who lose their sears. I have a registered receiver M16, so I do not know if on a RR M16, the sear can be replaced.
With the M60, a known problem is the trigger group falling out, and the shooter not realizing that the trigger group is gone, and then losing the trigger group (apparently the trigger group is held by a easily worn retainer part). On an M60, since the trigger group isn't the registered part, you can just buy a bunch more sears and trigger groups (that's why I already have six sears, which are only like $25-50 each). On an M16, if you lose the sear, you are out $20-30,000. Scary thought.
I have no personal experience or knowledge on this issue. It is just info that is passed from an NFA dealer to me. Take it for whatever it is worth.