With the natures listed by you(adamant) Cacturne would need a minimum of 46 EVs in speed to outspeed Swampert.
For the EVs Swampert has in speed (starting with 44) you will need 88 EVs for Cacturne, and 84 EVs in Swampert, you will need 128 EVs for Cacturne.
I did all the calculating with the Showdown teambuilder.