As someone said, the MAD-4X would be your best start for a Marauder 3R since it has a torso that looks close to what you want, simply cover the SRM6 slots. The new MAD 7D arms that were released would be good for the 3R arms. I am using the Hound's AC bit to be the AC on the MAD that I am converting.
As far as the Warhammer . . . yeah, there have been several torsos recently that can stand in for the Warhammer. As everyone said the Hammerhands, which has a appropriate head and place for the SRMs- the SRM rack that comes with it is sideways however with its flap. Other good representative torsos IMO is the Lament, its a bit shorter with a wasp waist but deeper though harder to put the SRM rack on. The Thunderbolt 1C torso can also look like it, it has a better slope on the upper front and the waist does not narrow quite as much as the Lament and has secondary weapons on the torso. It is also very easy to mount the SRMs on since it puts a LRM rack on the torso anyway.