That crack is definitely repairable. Plastic weld is going to be the best option.
Here's a post from my R129 restoration thread about my front bumper repair
Update: Bodywork restoration is well underway. As I mentioned earlier the car had damage on both rear quarter panels, along with numerous small dents on the top left by the antenna. Front and rear bumpers are off the car and being repaired as well. Next up the scrapes on the passenger door and...
www.benzworld.org
Jb Weld bumper weld works well too-originally my bumper was patched together with that and I cut a piece out of a plastic bucket as a backer.