If you use an impact driver first choose the correct size bit and tap it into the scew head on its own so it is good and tight in there. Then add the head of the impact driver and if it helps insert a socket extension between the head and the main body of the driver.Usually half inch drive. This gives good control in awkward places and allows you to get it lined up at perfect rightangles if you see what I mean. Load up the driver turning it the way you want to go and give t a solid thump with a club hammer. I have done lots of screws in this way it rarely fails.
Its all in the wrist action as they say so good luck.
This looks like the stock early CB750 flange screws to me. If so, the pipe will truly slide off the flange like he said. The best way to get these screws off is to use an impact hammer driver but you might have the best luck if you remove the front wheel to give you room to swing then put an extension between the bit holder and the driver to get it clear of the pipe and also give you a straight shot at it. You might use a large punch with the tip as big as the screw head and smack the screw ( HARD!) to shock it first and work the WD-40 into the threads. The impact driver needs to be exactly in line with the screw, then give it a good whack. It takes a good sharp whack to get them to budge!