hello Ric and sorry for this late answer, i'm not very often around here as I used to be.
I'll continue in in English if you're more familiar with it ...since I do not know where you're from ?
or in french, up to you.
you have to know that the mecanisme of your clutch is located and fixed to the thick heavy metal disk output of the engine and that if you want to put a clutch with a different diameter, the mecanisme will not have those fixations holes with thread at the right place and not the locators either.
you could then think it's just a matter of finding the right output disk that goes together with the clutch.
but then you may encounter another problem : the crankshaft on wich it is mounted can be different from one sofim engine to the other and it may not fit either.
I have had already that issue once:
I had a brand new engine iveco 4x4 with a pushed clutch with the 265mm diameter and I wanted to fit it in another iveco 4x4
at the time I didn't even know that it existed some pulled clutch on those, and that one was one of them with diameter 240, so the small clutch wouldn't fit to the output disk of my brand new engine !!!
I had either to change the gearbox as well to fit a pushed clutch... or change the output disk...since I had the one from a pulled clutch 240... but to my surprise it didn't fit the cranckshaft !
last option was to check if it exists pulled clutch 265 at iveco... and it does, I left in an iveco garage my output disk so they could surch for it, and in the mean time I've been lucky enough to find one brand new for a little price.
but in your case it's different because you want to fit a bigger one without changing system or engine... so you can always try to find a output heavy disk for the 265mm one but make sure the number of teeth fits with your starting engine and that it fits to your crancshaft.
that's all I can say about it.
If I find the topic where I already put pictures of those i'll put the link here.
I think on top of it the last time i talked about that was for an english talking person too.
that should make it easier to surch for it too.
here it is, that doesn't help much you just get to see pictures of both mecanisme and clutch disk but not from the output disk of the engine.
viewtopic.php?f=7&t=2958&p=33304&hilit=clutch#p33304
but reading again at your mail i see that you understood already the need of changing that thick disk...just one thing is not sure: will it fit the crankshaft ?
maybe you can see the references of the clutch on the picture...it's quite common around here and i don't think you would mistake if you go for a daily pushed 265 clutch from those years.