If it's on the original gearbox then it'll most likely be an LT77 gearbox with a short bellhousing, and the Discovery gearbox bellhousing is longer so you'll either have to move the engine forward or do something else. Depending on the engine you've got the bellhousing bolts should line up.
Pictures or knowing exactly what you've already got will certainly help here. You might already have a Discovery gearbox in which case it'll be a lot easier. I fitted a Defender R380, a Discovery LT230, and a Discovery 200Tdi into my 110 and it was fairly painless although I had to put in different engine mounts anyway and move the engine forward to allow it to all fit easily.