The Salisbury axle has been around and used on so much for so long, i wouldn't worry about it not being in production any more. I mean, neither are Series 3's, and it is a strong bit of kit. The competition lads go for them to fit to SWB's becouse they are harder to break, even under V8 strain, so I wouldn't have too many quarms about them.
But, does stick you with the stock 4.7:1 crown wheel ratio.
But, that's no great handicap. There are better ways of upping the gearing, if needs be.
Personally, a 2.25 struggles to pull the stock gearing, so there's not much to be gained from going higher ratio.
Now I know that you are planning on going Prima Td, but think about this. Prima motor, while it makes 30% more power, lacks the low down grunt of the 2.25, especially at or near tick-over where theres not enough gas to get the turbine windmilling.
So, if you upped the gearing, you would suffer badly at crawling pace where you just cant get the motor to make motive force that slowly.
Ie not so good off road or towing, or loaded.
Keep the gearing low, and let the thing spin, and you have less of a problem, as the motive force will be 25-30higher for the same engine speed, and as at the same road speed, the motor will be spinning faster, you'll have 25-30% plus at the wheels.
And, on the road, remember the stock motor will rev out at 4000rpm. I think that the stock transfer box ratio is 1.2:1 reduction from the crank, multiply that by 4.7:1 reduction at the diff, and a 30" wheel, and theoretically, a series three Landy is geared to pull 100mph at the rev limiter.
Yeah, in your dreams!
But that means that the stock motor is toping out at about 2500rpm, which actually doesn't sound that unreasonabe given the power it makes and the shape of its torque curve.
Prima motor makes more power and more of it higher up, and it will rev 25% higher to 5000rpm. On stock gearing thats a theoretical 125mph!
Realistically, the extra grunt is probably enough to get it up to about 85-90ish, which on stock gearing would be around about 3-3500rpm.
And you'd still only be pulling about 2500rpm cruising at 70.
If you were to bump up the gearing by 30% going to 3.5:1 crown wheels on Rover diffs in a SWB axle, then you would be geared for a theoretical 165mph at the rev limiter.
40-70 in top would equate to 1000-2000rpm - you just wouldn't have enough power that low down the rev range to pull those speeds, you'd be driving every where in third!