I am adding a 14-50 outlet in the garage. Running NM-B 6/3 Romex to existing breaker box. There is room for a 50AMP 240 breaker. I have 200A single phase service. Sheetrock above box. I can drill a hole in the studs above the breaker box and feed wire into breaker box from attic. Because of the existing sheetrock I don't have access to put on a clamp with a nut on the top of the breaker box (without cutting sheetrock). Can I use a simple bushing? I think I need 3/4" or 1" bushing or clamp. The electrician that helped me gave me a 1" bushing. Is that code? I also see snap-in clamps. That would work if I can find one. Do they make 3/4" snap-in clamps? Does code allow the simple nylon/plastic bushing where the wire enters the breaker box? What are my options?
You'll need a 3/4" snap-in NM clamp that can be installed from the inside of the box
You do need a NM clamp (vs a mere bushing) here, first off; a simple bushing does not provide the clamping of the cable jacket required by NEC 312.5(C) for cabinets or 314.17(B)/(C) for boxes. This NM clamp also needs to be installed from the inside of the cabinet, and that limits your options; in fact, the only product I know of offhand that fits the bill is the 3/4" Arlington NM95X, which maxes out precisely at 6/3.
I am afraid you do need a clamp. There are clamps for smaller romex that install from inside the box,
but I don't believe they are made for larger sizes like you need. and @ThreePhaseEel indicated in the comments there is a larger one available for 6/3 Romex
Arlington NM95X (see cables allowed below - your 6/3 is in there)
With these fittings, you fish the cable inside the wall into the box, then slide the fitting up the cable and snap the fitting into the knockout (hole). You have to be careful the sharp edge of the knockout doesn't damage the cable jacket while until you get the fitting snapped in.
Cable Ranges: One 14/2 to 6/2, One 14/3 to 6/3, One 12/4, One 12/2/2, Two 14/2 to 10/2, Two 14/3 to 10/3, One 14/2 and One 12/2, One 14/2 and One 10/2, One 12/2 and One 10/2, One 14/3 and One 12/3, One 14/2 NMB-PCS, One 12/2 NMB-PCS, One or Two 14/2 to 10/2 NMD90