Any chance there's a drilled/tapped hole in a similar position on the opposite side? If so, then I'd consider the "mirror" location on the side you want to mount to as a prime, low stress area.
A flash stud welder would be just the thing for this job - low heat affected zone.
Could you bend up a "U" shaped bracket that is a close fit to the top and sides of the arm and epoxy or RTV bond it to the arm and mount your block directly to the bracket? Just my $.0002.
-Jim