First of all, why do you want a Poison Type Pokemon? Doesn't that overlaps with your Venusaur, and also why not Nidoking?
It's found really early in the game as a Nidoran♂. After evolving it into a Nidorino, you can use the Moon Stone to evolve it into a Nidoking. It also really helped me against Lt. Surge, but you have a Venusaur, so I think you have already defeated Lt. Surge.
Ok back to the question, so obviously Muk is better than Weezing. It having a base stat total of 500 and having more Attack, HP and Special Defense to tank most of the hits. However, if you want to have good Defense, then Weezing has a really good Defense stat of 120. But I still think instead of having a Poison Type Pokemon, you should have a Ground or a Fire Type Pokemon. Or, you could just use Nidoking and you should probably replace Primeape with the Hitmons, [Hitmonlee and Hitmonchan].
I hope this helps!