1965 Dodge Dart stock differential advice

Nice car! IMO, there's not a thing wrong with going to the 8.25. I mean, it won't be original with any other rear but the 7.25, so it really makes no difference. The 8.25 will be much more budget friendly in any event. OR you could try to find a 7.25 sure grip unit. That would add a lot of strength to what you have.