Mk2 inner cv joints on mk1 drive shafts

Hi
I have mk1 with mk2 16v engine, I replaced gearbox from another mk2 only to find the drive flanges are 100mm but my inner cv joints are 90mm. Can I just buy the100mm cv joints instead of trying to change the drive flanges
Thanks

If you have 90mm spindles they won't direct fit as Grindage is involved.

A 100mm outer spindle will directly bolt a 90mm axle, all you have to do is exchange the inner drive flange.

A 90mm outer Spindle will not fit a 100mm axle.
Take the flanges off you old tranny, Use new seals, and wally, ya got its made and you can use your old axles.

I swapped a 90mm Tranny into my 93 Cabriolet that had 100mm axles, since the Axles were bad I had them rebuilt as 90mm, they worked just fime
and I never snapped one or broke it.  I would occasionally run the beastie at 100mph with no issues.

Just noticed my 90mm flanges have a little step in them the 100mm ones iv just removed don't, is this normal? Cant see a step in the diff for it to locate on, still seems to slide on ok.

Also some people say add an extra half a ltr of gearbox oil through the speedo cable after it comes out the fill hole, should I do this or not,
Thanks

I would not add an extra 1/2 L of gearbox oil.

When I fill mine I do fill it via the filler hole on the side of the gearbox and then jack up the passengers side a little (wheels still in contact with the floor) and dribble in a little more oil.

I also use fully synthetic gearbox oil and change it every 50,000 miles.
I've owned the car for 29 years and the gearbox has done 270,000 miles and still working well with using the oil and method I've mentioned.

Link to oil, 2L is plenty as you will struggle to get 2 bottles in the gearbox.

https://www.eurocarparts.com/p/triple-qx-ep-75w90-transmission-fluid-fully-syn-1-litre-522776022