youve got the right idea, but your geometry if you follow this plan will be thoroughly fucked. the arm is in fact triangulated to the ball joint. so to lengthen the hypotenuse (the forward side) as well adjacent side (rear, outward facing member) without adjusting the angle where the two sides meet the pivot shaft will only make the two came together at the same exact spot. now to accurately lengthen the arm's effective length the angles at the pivot will have to be determined to compensate for the extra distance. if you had the blueprint, or even better a CAD drawing, of the arm it would be simple geometric equations. but since you don't, or i don't think you do, the easiest way would be to reconfigure those two members entirely. but at that point why not upgrade to a replaceable ball joint like are in most aftermarket arms, since it is only the act of welding in a threaded bung, then screwing the joint in. that brings us to the next weak point, the pivot tube(s). since everything else has already been replaced, why not these? and since youre in there, use heim joints instead of bushings for ease of construction, plus the added benefit or adjustable caster.
so, after youve done even half of that, you might as well replace the whole thing. plus welding to yamaha factory steel is scary, like using butter to solder, just not strong.