Probably would be better to go rover V8 over 308 just because they are far more common these days. Datsun parts/engines in general are getting expensive, would make the suggestion if you were to go that way look at the d21/Navara engine perhaps, a z16 even should be around 100hp and shouldn't be hard to find. Maybe the viva breaks and diff would be up to it, I don't really know. Lots of great engines out there.