I had the same problem. Pretty common on those tall adapters. Not cheap from the dealer either (the only place I could find a new one). I get a discount and it was close to $200. My original was a 1980 and I noticed a slight difference in the new one (seemed beefier without the hollows of the original). Originally I had poly t-case mounts and rubber motor mounts. I put the ORD motor mounts in and kept the poly t-case mounts and no troubles so far (the 350 has been replaced by a 700 though, but the adapter is still the same).
You could have one machined out of steel (not sure on what a local shop would charge). I have thought about it myself and discussed it on here before (try searching), but making it stronger would only make something else the weak link (and I fear it would be the transmission housing). People have discussed using an L bracket like the old style 205 on the pre 73 rigs to help out in this department, again debatable on what the effects might be (good vs. bad).