It's not often you see a jza-70 in the SA trading post (but I haven't looked for a while)
Because I did the manual n/a MA-70 to 1jz powered MA-70 conversion
You'd need a manual front cut, and there is a bit of work involved so you'll have to weigh up if you want to do all of that. There are a lot more parts/mods available in SA for the Silvia than for toyotas, but if you want something a bit rarer, probably faster, and more comfortable go the Supra.
The n/a manual has a LSD G285 diff (from memory) which has a 4.3:1 ratio. Mine has been going for a year with the 1jz (and R154 gearbox) without a hassle although I don't thrash it that often. I suspect it sometimes slips more than it limits too (and one black line is a bit darker than the other)
I haven't had it at the drags yet, and it runs virtually stock boost and airfilter, so I can't comment on the times you'd expect. It goes pretty damn hard though and will step sideways/lose traction when you hit second in a drag. Losing traction can be a bit of a problem overall!