Agree with @
Fiordland, as long as your first flight is on time this should be no problem.
WestJet will arrive in the E gates in LAS, and the train to the D gates where Delta is departs from between gates E7 and E8. Should be very easy. No need to clear security or customs at LAS.
You don't mention if you're checking a bag ... I would recommend trying to travel with carry-on only if at all possible. Transferring between airlines, especially at LAS where such transfers aren't going to be common, increases the risk of your bag going missing by a fair amount. If you do check a bag, definitely check in with the Delta gate agent when you get to the gate and let them know that you should have a bag coming from a WestJet flight, and show them your bag claim tag.