I have an XCR with modifications similar to what you are looking at having done. Unfortunately, I didn't have the work done myself, so can't offer much in the way of guidance other than some photos which will give you a better idea of the finished product. I purchased this rifle on the EE about a year ago for $1500 CAD. It is a restricted , 16" U.S. version, imported by the previous owner from California. I know this only because I had to remove a "bullet button" which is unique to that state. But I digress...
As you can see, the new holes in the receiver mimic the existing vent holes, versus MLOK-sized ovals. The mods to this rifle predate the rise of MLOK. Note that the previous owner left the entire bottom rail. I'm not fussed either way, although in the interests of further weight savings I wouldn't complain if it were reduced to just a small rail at the front to match the sides.
I'm glad that the previous owner went with the LWRCI-like spiral fluting on the forward part of the barrel as I really like the look. The rear portion of the barrel under the receiver is longitudinally-fluted. The parts were refinished with what appears to be CeraKote following the machining. The entire upper receiver was redone to match the anodizing on the lower and the barrel was done to mimic charcoal Parkerizing. A subtle, but distinct difference between the two colors, which is very convincing.
And without further adieu.....