It will go.
I used a td5 out of a disco into a series 1. Use disco gearbox and transfer box , The engine will need to be back tight to the bulkhead and the headlight centre panel will need to be moved forward. Lengthen front propshaft, shorten rear(mine is 16 and a half inches but try and keep it 15 +) re jig centre g/box tunnel.
If you buy a mot failure disco use the ecu etc.
Only trouble I had was the series 2 disco I used had a factory immobiliser, had to send the ecu away to be sorted and also have a small box and loom fitted £75 .
My series 1 had been highly modified back in the late 80’s and registered as such.
Been asked a few times…Why?
Well because born in 1951 I drove series 1’s and I didn’t think they were great (my opinion) my series is now usable , anyway it gives the rivit counters something to talk about.