If you have no plans on converting it to full-auto with a drop-in-auto-sear, then the sear block will have no detrimental effects at all. Some people say the sear block unintentionally prohibits the installation of a match trigger as well, while others claim that it does not. My pre-ban Colt lower has the sear block and I have no intentions of removing it. If your lower has the "large" pivot pin, then you will need an adapter to mate a "small-hole" upper to a "large-hole" lower. If it has a sear block, it probably has the small pivot pin and they will mate up fine. All rear takedown pins are "small". If the pivot pin is the same size as the takedown pin, then it is a "small-hole" lower. Hope this helps.