I don't think I rate as a seasoned 'Tiger guy' ( yet, lol :O ) so I'll only give my 1¢ ( 1.31134 ¢ Can. )
As far as trans and rear end, I think that if you're lucky enough to have those 2 items come with your car, you might as well keep them around even if they're not actually 'in' the car ! They've been together this long so why seperate them now...

My car came with the original #'s toploader and I really would have liked to keep it 'in the car'... but... the car also came with the original #'s rear end (housing) that had a Stude' 3.54 gear set and 'twin traction' LSD in it so for the sake of driveability , I put in a Tremec 5 speed Z-spec trans . The toploader has now been on the shop floor for a few years and unless I could get MKII trim money for it, It will be in my possession as long as the Tiger is! ( and won't be reinstalled by me, ever )
Now.. for other Tiger' specific parts... you know me as a 'modified' guy and the P.O's and I have changed many things on my car... I've sold ( and given away ) a whole lot of things that were 'Tiger' specific . My reasoning was that there are a whole lot of 'concours correct' guys out there looking for parts and I totally respect them for what they're doing.... If I can help keep them on that path, I will... if I had a second Tiger, it would probably be on the same path

Now, as far as keeping all those 'bits' for the next owner... I highly doubt that anyone that's looking to put a 'stock original' car together would be interested in buying a car like mine ( or Duke's for that matter ) and 'put it back to stock'! That just wouldn't make sense
P.S. +1 on everything that Randy said!