I've done the 200/300TDI, 2.8 Cummins, OM617, and the 4BT swap into RR Classic, Defenders, Disco's, and even a Lightweight for different clients over the years and the most bang for the buck in the end was the 4BT if you can handle the rattle of an old style mechanical diesel. 4BT/NV3500/LT230...