[land rover series 3 gearbox]

Hi everyone

Im looking for some help we have just brought a 1972 series 3 that was a petrol and was converted to diesel at some point the problem we have is that its top speed is about 40-45 mph is there any thing we can do to make it go faster or was there a gearbox for petrol and one for a diesel sorry if we sound silly just would like to no .

thanks

yours

andy