I've done this in the past, both on a 400 and a 500.
The 400, you'll get about 600 out of, peak.
Keep the carrier about 100-120 watts. Maybe slightly less. I prefer 1/5 carrier / PEP swing, especially running a mod box.
The 2879 pretty much saturates with CLEAN power out at 15 watts input. The 2290, in class AB, with about 12. SO, at about 50 PEP, you're going to not see much increase in output, just make the meter move farther to the right, if that makes any kind of sense....
If you have the mod V, turn the variable to the point you get about 60 on a AAAUUUUUUUUdio. Then bring the deadkey in your radio up to the point where you're deadkeying the same with and without the mod box on. If you have the Mod +, then hit the yellow button in, mine does about 55 watts that way, 85 with the button out, unless I'm running one of my assym mod radios, then it's a flat out 85 no matter what...
Again, then turn the radio to where the deadkey is the same mod box on or off. There is a variable cap inside the mod+ to affect this 'modulator' effect, you can pretty much use that cap to set the mod point at anywhere between 1 and 4 watts of carrier... Each box slightly diff, but you get the idea.
Set up that way, your 400 should still like you
--Toll_Free