So I read the PDF. Did you essentially do a rear end swap (OB cage, OB brakes, OB hubs, OB axles, and OB swing arms)? And retain the IB diff? Does the OB cage bolt on the early car?
The cage is the inboard setup that came with the car but a later outboard cage should bolt up. The swing arms are also the original inboard setup. I’m using XJ40 hubs, axles, and brakes. The pivot point where the hub attaches to the swing arm is slightly different, I believe because the XJ40 is metric and the older XJS is not. They will bolt up but some fiddling can be required to make it work.
I chose to fabricate some hard lines that run from the original distribution block out to the hubs and tied into the XJ40 brake lines.
The easier solution is to just find a complete rear end out of a later XJS but I couldn’t find one for what I wanted to spend.
Hi Bob, I don’t have an XJS. I’m working with a 68 420G and want OB brakes and better pats availability. Gosh I hope my big boat’s handling won’t suffer too much when losing the IB brakes 
I know at some point the diffs went from Salsbury to DANA and would prefer the later. Did you have both with all the parts? Comparing 70’s era XJ dimensions and the 420G it looks to be a near bolt in. Are you saying an XJ40 OB complete bolts in directly in place of an early XJS? It would be nice not to have to adapt parts. Although I guess the mixing and matching has to to do with the shock absorber mounting. maybe a hybrid in needed. You stated you wanted to retain the early 2.88 gears. i would favor the 3.54 gears. Do the DANA diffs mount the same as the Salsbury? I’m thinking for me, IB cage, swing arms, and 4x springs Dana diff, OB axles and hubs with machined pivot adapters,.
The XJ40 set up doesn’t use a cage and is quite different to the older IRS. I don’t think it viable to swap the whole thing. As Bob says, the hubs, axles, and brakes are the only things you need. I was lucky enough to get a complete 1994 rear end so didn’t have to mess about with too much fabrication - especially if you want a working handbrake. But I did reuse my original cage and diff. I believe the diff swap shouldn’t be an issue, plenty of info if you search the forum.
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Basically it does.
The shocks bolt up to exactly the same places as before the conversion.
I actually did want the 3.54 I just couldn’t find one reasonably priced. That did change and it now has the 3.54 diff. It bolted right up.
Is there a year of 3.54 XJS that has the original cage and 4X springs with OB brakes?
I thought that all the US spec inline 6 cars had the 3.54. Everything else should be the same. Outboard brakes I think came with the end of the facelift cars, 94-96? Someone will pipe in with the correct information I’m sure.
Correct. The V12s with the 6.0L V12 and the GM 4L80e transmission and the 6-bangers have the 3.54
Not sure how you would search for it, but I think someone on this list figured out that their VIN was something like 10 away from the changeover point, unfortunately the wrong way 
My OB donor car was a 1994 6 cylinder. I’ve kept the 3.54 diff for when I do the manual conversion.
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A source I put some faith in says the outboard brakes came at VIN 188105, which was May 1993
Cheers
DD
That wasn’t it, but it’ll do
Cheers
DD
Yep min 188097 lol. Oh well 