What I would do with a TD is to make sure everything is as it should be, then determine if you need any more power. Injectors, valve clearances, clean air filter, and all the other general tuning jobs. Mine has the power to comfortably drive at a steady 60-65mph, and how much faster do you really need to go in an old Land Rover? A good TD should be perfectly adequate for most Land Rover type stuff. Of course, if it's knackered, it will be down on power and trying to increase the power by upping the fuelling will only hasten its demise.