moritz97,
Until you figure out a permament solution, you could try the u-bolt crossmember mounting method where you take u-bolts that fit around the frame and have the same width as the bolt pattern on the factory crossmember.
I have a picture of Marlin's old u-bolt crossmember mount, but right now I can not seem to find it..
Basically, the u-bolts can slide forwards of rearwards along the frame (in certain areas) which provides many mounting locations without any welding. Also, they can be removed in the future if you decide to change it.
If I find the picture then I will let you know
