My Block man obviously could not read blueprints and I am left with upstairs cmu rough openings that barely meet egress codes. Want window centered in block ( not attached to interior framing. Can I "notch" around the r.o. with a grinder and then insert the nail fins in the notch and then trim using same method I used to install downstairs wood bucks? R.O. is 48" wide X 40" high...... Window is 47.75 "wide X 36" high slider.... Will pour 4" bottom sill after install. Comments.... Especially if idea is dangerous, please
Your idea is workable although you may need to cut off all of the flange on one side, if not both sides. A window that wide that you mention is wider than that when it is placed in the opening at an angle to insert one side into the groove you refer to. The top flange, the really important one should fit between the lintels over the opening.
When you specify a window going into a masonry opening to the window supplier, they will be able to direct you to the proper procedure for that type of install. It requires a clips, 3 on each side to the to the window for the holding of the window securely in place. These clips screw to the window jamb, and extend to the inside of the rough opening so the extended end can be attached to the block. You can use concrete screws for this. Drill the pilot holes no closer than 1 1/2" from the corners so they don't break out.