I'd still stick with a 2.78 Blocker behind a 4.2L and trimatic as long as the engine has been returned to its un-pollution state like a HQ-HJ. A 3.08 would be OK too. Any lower and they rev too hard especially if you are fitting lower profile than standard 14" tyres. 2.78 and 2.6:1 gears and centres are far easier to find especially LSD. 3.08-3.55 gears and centres are far harder to get.