Yes, although if you use the disco R380 then the gearstick won't be in the "defender" position, however i've been in a couple of converted 110's with the disco box, and it's axtually quite comfortable driving with the stick slightly further back (they also used the disco shorter gearstick)
You'll need to weld in the 300 engine mounts as they're different to the 2.5 (PM me your email and i'll fling over some dimensions of where the brackets go)
The gearbox mounts onto the removable crossmember (which just bolts up into the chassis, all the holes are there on any 90/110 chassis)
Get as much as you can out of the disco... exhaust downpipe, fuel pipes rad/intercooler, header tank etc. it can all be made to fit. and the PAS box too! (and hoses)
even the front axle (if needed), expecially if the disco has vented brakes.
