cracks should scare you. I stretched mine 4" simply for appearance. I dont do alot of racing around tight corners. I did not cut the arms. I think it could, and would be disastrous. I believe compromising the integrity of the original arm is a bad idea. I gained my length on the bushing end. I measured the original swing materials using my digital caliper, and ordered the new steel to the exact thickness specs. (Welding 2 pieces of different thickness material together is not ideal.) A good and trusted friend at the machine shop cut and fit the steel, perfectly. My best friend, who is an educated welder w a certification did all the welding on my patio while I watched, double checked and measured. I was not about to blindly trust "god knows who"...If he or I had any questions about the entire project it wouldve been scrapped immediately.
The swing arm floats, so it really does not support any weight per se. The weight of bike and rider is transferred from the frame through the resistance of the shock springs to the axel, bearings, hub, up through spokes, down around rim, through the tire and to the ground. HOWEVER, Even though the arms are "connected" to one another through the axel. The arms will twist and flex a bit. Due to the fact that each shock is somewhat independent of each other. This twisting and flexing minor as it may be. eventually will cause breakage at the weakest link...anyone care to take a guess where? At the EXACT point these guys are cutting and welding. Espeacially if the welded parts are of different thickness. The heat needed for good penetration on the thicker steel will harden the thinner. Causing it to become brittle. OR, if its welded at a lower temp setting for the thinner material, it will not penetrate the thicker deep enough. The bead will literally just be sitting on top of the thicker metal.
Also there are more pressures placed on the arm. Pushing and pulling. The arms are constantly being compressed by the force of the rear tire pushing the bike forward. Not to big of an issue with these small lightweight bikes w little horse power. However, The pulling apart force generated every bump and pot hole we hit. could be tremendous. Pulling that arm free of the rest of the bike and you. If one cuts and welds those arms I wonder where its gonna pull apart, after hitting that pot hole at 80 mph while passing that big truck.
I can write alot more...If you like? Its your ass on that thing. I recommend you dont do it.
Heres how I did mine. http://www.dotheton.com/forum/index.php?topic=28306.0