Well what a day !! .. It's taken me aprox 7 hours to change my diff in my Seiw Westfield, from a 3.64 LSD to an open 3.92
I got the old diff out, dropped the new diff in, couldn't get it to fit, didn't matter what way I tugged or pushed it, after an hour of scratching my head & turning the air blue with expletives I realised the diff I had taken out had the casing ground on the left hand side to clear the chassis !!
After going through probably 10 grinding discs on my dremmel I had ground the necessary off the casing
Doing the job single handed wasn't easy
But.....its transformed the car, and that's no exaggeration!!
I was a bit concerned incase it made the car under geared, but with the BGH gearbox its spot on I would say
It was at 3400 revs at 70mph; it's now at 3700 at 70mph, I can live with that & the difference in acceleration !!!!!!!!!!!!
BIG thanks to Terry Everall for selling me a cracking condition diff 😃

Couldn't you have swapped the crownwheel/pinion from the open diff to your od one so you'd have retained the LSD?

^ wot he said, it took me a couple of hours to drop a 3.14 CW & P into my LSD

Had to make a long socket, just cut the top off an air socket and welded 4" of scaffold between the parts.

Interesting to hear that is still driveable and an improvement. I have been struggling with this issue as well. My heart says I WANT a 3.77 diff (English diff) with a quaife atb and a long first heavy duty type 9 (or maybe go for the rx8 six speed conversion), my head says I NEED the current open 4.125 and the standard type 9 to get it on the road and drive it (upgrade later). Based on ypur observation I should be pushing 3.9k at 70, assuming the same tyres.

On my old westfield I went from a 3.9 open to a 3.6 LSD - I MUCH PREFERED THE 3.9 OPEN DIFF!!!!.

I went from an open 3.62 to a LSD 3.9. It did indeed make the car much more lively and better to drive, it also strangely made me hanker after a gearbox with better ratios than the standard type 9.