Couple of tips:
Get one that has the buffer tube hole roughed in. Use the best reamer (floating Shell type) to get the hole right make sure it is straight them ream and tap. I find this the hardest part. Buffer tube tap is most important and specs vary get one from a quality supplier not a cheap one from the fixture folks. Spend the extra 20 bucks here and you will smile later. Make sure the hole is centered where you want it as well as tilt.
If not using a fixture to drill make sure you are square with the part it is hard to eyeball and that is not close enough.
Set the buffer retainer forward from the distance on the drawing and the front of your buffer will like you make sure the buffer will rest on the back of the carrier not the pin or it will eat buffers. Get it no more that 11 degrees tilt.
Most of the ones I have seen you are able to set the width across the trigger group to get it with no slack side to side so have your trigger group in advance. I cut the lower short of the trigger width and polish the trigger down to just slide in. Plan ahead measure well and don't assume that the hog out by the supplier is in the right place left and right. Where do you want yours is the important thing.
Drill the rear pin hole under size then mount your upper in the factory front hole and see where you are so you to get the best pin fit, tight by a couple of thous. You will be making the final top surface where they mate with the upper flat and the right height from the pin holes go slow with a new file lots of test fitting an checking will produce a very nice fit.
Deburr the inside edge of all holes something important goes inside outside deburr is for looks.
Use brand new high quality bits. Don't get cheap at this point and use an old bit once the bit goes through that is your hole. If not using a fixture use a spotting bit to place all holes. The hardest small hole is the bolt release pivot aircraft bits are your friend and spot it by hand first the surface your drilling will be angled I spot it with a dremel. Lubricate Lubricate Lubricate when drilling the holes.
If you get the fixture and drill the holes it will work fine but with a little fore thought you can get a custom fit and great performance. The set up is the fun and mind bending part the actual hole drill or tap should just take seconds set up should take hours. Go off and take a break come back and review the set up then drill or tap.
Good luck.
Oh one last tip:
Don't let your wife see the Amex bill!
