....or should I just buy the 10.5 upper? Thanks!

I believe if you chopped a 16" barrel you would also have to resize the gas port.  With all the labor involved versus the factory 10.5 I would go with the factory.  Not to mention if it does not run you can ship it back to LMT.  A chop job would not allow that.

If you are planning on using a supressor, having the 16" cut down and threaded to be concentric with the bore by a competent smith would avoid the possibility of bad factory threads. This will help minimize POI shift and help reduce possibility of baffle strikes.

There have been more than a few LMT barrels out there with muzzle threads that were not adequately concentric to the bore. They're close enough for use with an A2 FH, but people have had problems when using cans.

Just something to consider.