Stop bruising your knuckles and your ego, and take the easy way out. To remove the crank dog nut un-bend the locktab washer, install a large wrench (1-5/16" for the MGA and MGB) with a long handle. Put gearbox in neutral. Rotate crankshaft clockwise until the wrench handle comes to rest against the inner fender. Hit the starter for half a second, and the dog nut will be loose.

If the engine is out of the car, use a strong 1/2" drive socket handle which is shorter than a breaker bar. Rotate crankshaft to position wrench handle straight up. Press gently to the left (anti-clockwise when facing the engine) and strike the handle soundly right to left (anti-clockwise) with a two pound hammer. Click for larger picture. This is a Craftsman 1/2" drive ratchet handle I have been using in this manner since 1968, and I haven't been able to break it yet. Maybe they don't make 'em like they used to?

Addendum, July 2011: Okay,I finally stripped the ratchet after 43 years of serious use. Sears tool store replaced it under warranty, but the new exchange tool may not be the same quality. For one thing it has a plastic toggle lever for reversing. Time will tell.

Addendum, August 10, 2018:
On 8/10/2018, R. Michael Baiada in Evergreen, Colorado, USA wrote:
"I had to replace my crankshaft pulley on my 1600 MKII MGA engine because of loose rivets. I purchased a new cast iron (no rivets) MGB crankshaft pulley with damper, which is actually $22 less expensive and supposed to be better for the crankshaft than the original 3 piece, riveted crankshaft pulley. I ordered the front rubber seal and crankshaft lock tab, less than $4, so worth doing.
I removed the radiator, fan and fan belt and then tried to loosen the crankshaft pulley bolt. I tried all of the recommended ways to loosen it (breaker bar, breaker bar with a 4-ft pipe extension, blipping the starter, etc.) with no luck. The only other method was a 1/2" impact wrench, which I rented from a local equipment supply house. But there was no way it would fit with the oil cooler above the horizontal radiator duct panel and the oil cooler lines below it.
I then purchased 26 inches of 1/2" drive extensions (a 20" and 6"), and slid it through the MGA hand crank hole in the bumper and into the 1‐5/16” socket. I was then able to connect the 1/2" impact wrench in front of the bumper and the bolt loosened right up. I didn't even need to put the car in gear.
As a note, depending on the socket, just break the bolt loose with the impact wrench and use an adjustable wrench to remove it the rest of the way to avoid jamming the socket into the steering rack with the impact wrench. I also removed the spark plugs and spun the engine to put the Woodruff key at the top of the crankshaft to avoid it dropping out of the slot. My Woodruff key was tight and in good shape, so I left it in the crankshaft".