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Seattlejester makes a solid point about tires. I've broken u-joints in the factory shafts at 230ft-lbs on big ol sticky tires. Drag launches and clutch kicks are what break diff parts, so as long as you stay away from that, and lower than your 450ft lbs, the smaller diffs will be fine. 8.8/9.0 Ford diffs and R230's all require some level of fabrication to fit. The easy move here is to keep/get a R200 rear end, and put a LSD build for a s13 open diff in, and then buy the Wolf Creek CV's. That's my setup, and it's held up amazingly well. With this option, you don't have to source the Subaru to Datsun adapter stub shafts, or the increasingly annoying to find Subaru R180's. There are a couple threads on here about using the OBX helical diffs if you want a real cheap solution, but you can also just leave the factory diff alone, and add in a LSD later if you aren't sure you want one now. Some more info about your goals would be instructive here and we could give you a better answer!