You could bolt or weld on some steel plate in the recess of the Rail, then weld the top to that to get a bit more purchase on it, if you are worried about it being thin. This would add to the overall mass too.
My forge was nothing like big enough to do mine, I build a bonfire and put the rail in the middle of it, after about an hour it had reached the right heat for heat treatment.
I never bothered to temper it. But it seems perfectly fine and like I said has taken a real beating.
Yours is a much better design bionicarm, so it should be very useable.
As for hardening, maybe it would be a good idea to slice off a couple of pieces and harden them both, one in oil, one in water then test them out. Any dents or chips would give you a rough idea what to do with it.
All the best.
Andy