Stick to your guns, Ted.
As Greg just said, the only thing that really matters is whether you can make a good-enough case that the alloy arm existed in 1977 for 1977 models. If you think you can do that, then that's all that matters.
Based on what is in the rules, the rest of it WILL go your way if push comes to shove. The stuff about availability, the style of the weld, etc etc are all going to evaporate very quickly once it becomes about the rules, rather than opinions/gut feelings/etc.