I've never welded a 40' but I've welded the knife end of ours on more than one occasion, its a 35' double drive so 17.5' long sickles.   

What I do is take a new pair of sickle sections and weld them together while on a good spot of the bar to get the spacing correct.  Then V out the bar so you get good penetration, then bolt the pair of welded together sections over the break and weld the bar and sections all together.     The sections serve as reinforcement.        I know it doesn't flex very well there after welding it all together but on the drive end it really doesn't need to flex, flexing is why it broke in the first place.