Sorry Pepper, have to dissagree.
Rebuilding the SIII box doesn't need any tools more specialist than a decent pair of circlip pliers. There's nothing in there to be pressed in or out, and the only really fiddly bit is assembling the syncro hubs, and thats only becouse teh springs keep wanting to ping out when you push the retaining ring over the top and push it past the detants!
Most awkward bit is splitting the main and transfer boxes, as some of the bolts are 'hidden'. But, clean the thing up well before you begin, and follow the Repair Overhaul manual; apply a lot of good 'house-keeping' to your work area, by way of having lots of pots and jars and tins to hand to put bits in, and labelling them so you know what they are and where they came from (Digi-cam is great too, take pics at each stage, and it helps you see where stuff came from when you come to put it back together), and applt a lot of patience and perceverance, and the results can be terrific, and the best VFM you can get.
As the gear-box and X-fer are the 'heart' of a Series, and THE most difficult bit to get at and do anything to, doing it once and doing it properly makes a lot of sense.
Engines, hanging off the front, you can afford to lob in an unknown quantity and risk it going pop and having to swap it out again, as you dont have to take off much besides the bonnet to get it out; to get the gear-boxes out, you need to take half the car to pieces, as the seat-box has to come out, and if you have a hard-top, the doors have to come off too.
Back to the VFM thing; an LT77 from a Disco wont have the right linkage on it to suit a series, so youd need to pay extra and get one from a defender, or try and source a defender gear-linkage; THEN you have to worry about the transfer box, and either using the matching LT230 and working out how to get the gear ratio's right again, and cope with permenant four wheel drive, or mating it to the series box... AND modifying the floor to suit new lever possitions.
A 'take-out' LT77 from a Defender will probably set you back best part of a couple of hundred quid; add to that the cost of an Ascroft adapter plate to match it to the old, and still as worn series X-fer, you'lkl STILL have had to strip down to remove from the Series box, and you aren't saving yourself much money or hassle.... OK so you get a fifth gear, but you still have a well worn gearbox.
Ascrofts 'exchange' boxes are very good value for money; they AREN'T cheap, but they are a genuine remanufactured box, with about as much life in as you can get, and built to a standard by experts.
But, if you DIY it with the parts from Ascrofts, you stand to save over half the price, for pretty much the same thing.
NOW, rate the alternatives:-
A gear-box from a 'scrap' series could cost anything from £50 to £250, depending on where you get it, and how hard you bargain, and there is little garantee that it has much, if any more life in it than wot you already got. But if needs must, may be worth the risk, JUST to try and get mobile again, especially if you get one for closer to £50 than the other end.
At the higher end of the budget, £250ish, you start seeing adverts for 'exchange' boxes.....
Now a little while ago, Paddocks & Craddocks had between them the last of the Nato Reserve stock from clerance, and were offering boxes for that kind of money; but with the disclaimer that they could have been brand new off the shelf, never been used boxes, boxes removed from knackered landies, which may or may not have been reconditioned by bored squaddies, or complete knackers taken out becouse of a box fault and simply booked back into stores to await rebuild..... thier caveot was if it didn't work, you could swap it for another one.
Which was a pretty good deal.....
But think about it, theres over £300 worth of parts needed to properly recondition a gear-box, so anything offered for less than £500 isn't going to have many new bits in it, is it?
It will, most likely be a take-out from a scrapper, with little more than a steam clean, at the bottom end, and as you aproach £700ish, it might have been completely rebuilt, but as like as not with a lot of old bits from other boxes, or they wouldn't be able to make a profit on the job, would they?
IF you are really stretched for dosh, then my reccomendation is to go look for a take-out from a scrapped Landy. It may not last too long, and you may have to do the job all over, but as long as you can get one for under £150 then its worth the risk.
If you have the money, and dont want to do a DIY rebuild, then an Ascrofts 'exchange' (though I think that they actually work on your box now due to shortage of casings) is definitely worth the dosh, they AREN'T exhorbitantly priced, when mine went, in 2000, one of thier remanufactured boxes was well over a grand; now they are under that, I believe, and they represent very good value for a box that is pretty much remade 'as-new'.
In between, a DIY rebuild is your best bet.
As said, its no more expensive and probably no more hassle than trying to cobble in a non standard box, and gives you a box that looks like the drawings in the manuals, with a full compliment of life in it, that shouldn't have to be touched again for another twenty or thirty years.
The work is NOT hard, and doesn't demand any great level of expertise or specilist tools, just patience a little dexterity and a bit of organisation.
Its not a 'quick' job, as there are a lot of fiddly bits to deal with in there, but its possible for a novice to do a box in perhaps two week-ends, if they are contientiouse and organised.
As said, once you have the box out, though, you might as well do it all, which means main-box AND the X-fer box, and while you are at it, the hand-brake on teh back and teh clutch up the front, including new slave cyliner, spigot bearing and spigot arm bushes.
Its not 'essential' but, as said, its the heart of the vehicle and a ball-ache to get at, so, particularly the hand-brake, oft neglected.
And once done, you will be amazed at how 'sweet' the shift of a standard series can be, it REALLY does make a big difference, and as said, should be good for another twenty or thirty years.