You can fit a 200tdi mount on the 300 to save moving it but then you have to get a bend in the downpipe to miss the mount but theses are avaliable on line at reasonable price
I have a 300tdi and R380 in my 2.5td 90 but I used the LR conversion kit that I purchased from sawtry. You also need to think about the clutch being on the other side!!
Mine uses standard pipes so its easier if I blow one.
You can fit R380 without seat box!!
Fitting the R380 is much better as all the pipes are standard and don't need adapting.
Also engine sits in different place for each box so will be a pain to change after.
Are you using lt77 or R380??
If using lt77 you will have to remove a few studs to line up. And engine will sit right on bulkhead too, which means making up pipe work.