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.
There's two ways to do it. One is to break the baddies up into groups of 5-8 and assign them action cards that way and just keep good notes as the fight goes on. If you don't mind going farther there are rules for it in the adventure book The Unity. I don't have the file on me but I believ it just has the characters rolling what they would for attacks and using a chart and likewise for the baddies.