If you have a Series 1,2,3 or R6 and want to fit a stronger but not excessive engine, here are your options within the Land Rover range::

2.5 Petrol, 2.5 Diesel Turbo (Falcon) and 2.5 Tdi (200 and 300)

These  4 cylinder engines all use the same basic engine block as the 2.25 petrol and diesel engines in series 2 and 3 Leaf sprung vehicles. They are directly swappable on the gearbox, bar one or two extra studs (Tdi's). 

The LHS mounting is retained while the RHS mounting must be shortened when fitting a Falcon engine in a 4 cyl series chassis.

Land Rover V8 3.5 Petrol

As an alternative to either the 4 or 6 cylinder engines, the Rover V8 is a good choice. 
The engine is reliable and robust, quite light in weight and similar in fuel consumption. Less so than the 2.6 side valve. It is  easy to repair and rebuild.
All parts are locally available and are reasonably priced.
The left hand foot well needs to be cut to clear the exhaust. There are square type manifolds available that clear the chassis rails. The whole conversion can be completed without affecting the look of a Series 2 or 3 vehicle, but to accomplish this the front cross member must be cut out an re-positioned. If you are going for a flat nosed look (R6 or 110/90) you will easily fit everything in without cutting too much.

Adapters are available off the shelf from Gilo's adapters, BUT you will need to make a spigot shaft bush and thrust bearing spacer of sorts.