I have done this to my 110. I went from lt77/ 200 to 300/4hp22
You have had good advice here.
Avoid the disco t/box. The 4hp22 lockup and 4th is similar to a v8, so too tall for a defender.
Also, the standard cooler lines are too long. And IMO too restrictive.
Purchase a 30 row oil cooler with an10 lines off ebay, use m16x1.5 to an10 adaptors (male/male) and it will plum straight in to the box. Use 90 degree elbows box end.
It will flow better, and cool better.
Acceleration is as good as the manual if setup correct, offroad far superior and you can cruise at 65/70 as opposed to 55/60 whilst keeping the revs nice.
Fuel economy is good. Noticed no difference to my 200/lt77.
Heat and fuel economy will be better with the def t/box due to earlier lockup.
Totally transforms the defender into a semi refined drive.