My mate in 1982 was one of the first to fit a 3L V6 to his series 1, he still has it although like triggers broom, its had 3 engines and god knows how many gearboxes (Currently off the road due to the loss of 2nd gear!)
Having been in it doing warp speed up the rear of 2.8L Capri's who could not believe that a Land rover was keeping up, its a fun machine and will out preform the rover V8 in acceleration. But what lets it down it its tendency to try and spin in 1st & 2nd when off road! Also as also said parts are becoming rare.
The 2.9L is the way to go as the Ford Scorpio Mk II 2.9 V6 Ultima used this engine until 1998 so they are still out there and parts are all over the place!
Also for added suicide try fitting the Cosworth BOA 24V V6! As it used the same block as its 8V counterpart so the convention kit will fit............ this will give you 195 PS (about 192bhp or 143 kW) and 203 ft/lbf (275 Nm) at 4500 rpm in standard form........ try Turbo charging it and expect 500+ BHP and every component in the series drive train to expire in 30 seconds!
I know someone trying to fit a Ford Explorer 4L engine with 24v heads!
Good luck, light right foot will help!