I have seen a few people have the same issues and gotta wonder why the companies don't use thicker wall tubing or at least sleeve the first few inches. Is there any way you could run a brace from the headers to say the bellhousing? I would think its an issue with that style of header and how far out the weight is. Kinda like holding a set of dumbells with your arms extended. Do that too long and your shoulders feel like there on fire.
odd thing about it is that that crack is on the inside of a bend,where the tubing is compressed and therefore slightly thicker in wall thicknes compared to what you will find on the outside radius of the bend wich usualy is alitle thinner.
my thoughts on why the thinwall tubing is used is a few diferent points.
less steal costs less
thicker material takes more time and wears alitle harder on cutting equipment
thinner material is lighter in wheight
if the company is making raceheaders as well as typical streetstuff they already have the tooling for the thinner wall wich the racers prefer for lower weight this means they also wont need to buy two sets of mandrells and also cuts down on setup time since its not always a mather of just swaping the mandrel when doing a diferent wallthicknes,sometimes you need more or less preasure on the sliding (and or somethimes boosted woch may alse need adjustment on boosterspeed) suporting die.
having more diferent wallthickneses available also means you need more space and that you have to buy in less of each spec tubing meaning the price for each lenght will go up slightly, also having more tubing of diferent sorts on the shelf means that it may get old lying ther on the shelf, i dont know why but precision steeltubing seems to harden with time wich does make it almost useless when bending around tight radiuses, when the tubing has hardened by it self two diferent things can and will happen 1 it breaks while bending,this is a truly horrible senario that from time to time can take lots of time and effort to free the remains stuck on the mandrell it may also damage both the mandrel and wiperdie when this happens,its also very easy to damage the mandrel while cutting the piece of tubing open across the lenght of the mandrel 2 when the tubing does not want to bend since its to hard it may wrinkle in just the same way as a typical walker brand exhaust pipe,when this happens the wrinkels eats away material on the wiperdie as they pass over it and it very quickly eats the razorsharp tip of the wiperdie in the area wich is closest to the tangentpoint on the radius, it also puts an enourmus force on a very small area aproximately 1" behind the tip of the mandrel and puts a increadible force pulling the joints apart holding the ball segments at the end of the mandrel,these tools are kind of expensive to replace,but will last a very long time even under the huge forces they are subjected to as long as everything is done right.
puting a sleeve close to the end of the tubing would probably be dificult in a massproduction enviroment and would mean you would also need ot dent the tubing more to create space for the bolts. but i have seen some headers with a added bracket to the top of the flage wich is easier to install.
sorry this ended up being a mandrelbending 101 post but hope you get an understanding for why and how it works.
but a thicker wall tube would probably be easier to weld to the flanges to since it can take alitle more heat,its horrible to weld thick flanges to a thinn piece of tubing since the thick flange absorbs all the heat very quickly and dont want to melt while all that heat needed to penetrate the thick flanges easily burns thru a thinnwall piece of tubing
also TTI uses thicker wall tubing and i think dougs does it to but alot of people dont want to pay the added price
doesnt mather if you tell most people that there is better alternative than the cheapest one, and then they come back mad when the cheap stuff wont last 
