Well what kind of lever gun is it?
An antique Winchester?
Or a modern Uberti?
With the old guns, normally weaker loads should be used. I can't give you exact numbers, but if you buy "cowboy action" cartridges, you should be fine. Velocity will be slow, as the .44-40 is a bulky, somewhat slow cartridge, just as any old cowboy round is.
Here is what Wikipedia has to say about it:
http://en.wikipedia.org/wiki/.44-40_Winchester
Seems like a 200gr lead bullet at 1,245 FPS would be a pretty quick factory load.
Now, for the newer guns, you can load them much hotter. The metal is of higher quality and designed to chamber higher-power rounds. An example of this is the Model 1886 Winchester. The original gun can handle cowboy style loads, but the Browning reproduction is supposed to be able to handle loads of an additional 10,000 psi. A huge increase in the modern reproduction.
Just don't put too high power loads in the old guns and you should be fine.