Running pre-Reloaded Deadlands and my party may be encountering a large combat. I was looking, but cannot find anything that deals with 25-40 opponents.
There is a simplified version of the Deadlands mechanics for use as a minis game. It's called Deadlands: The Great Rail Wars.
It's often said to be the precursor to Savage Worlds, mechanically; it's even been called the stepping stone from Deadlands Classic to Savage Worlds.
Another option to consider would be to apply the group action rules from the similar mechanic d6 system.