I did some searching and can't find a prefabricated motor plate for a Gen III Hemi. Doesn't mean they don't exists, but I can't find one. I think your best bet may be to find a '73 and up K-Frame and use spool type mounts. The spool type mounts SHOULD be strong enough.
According to the Mopar Performance Parts Catalog the aftermarket Gen III Hemi blocks have attachment points cast into the block for a front motor plate. Given the nature of front timing cover on the Gen III Hemi, a traditional motor plate may be problematic.
From the pics I've found online, even the drag pak challengers use the stock motor mounts bosses. Though they may use solid mounts.
Here's a pic of some solid mounts that may meet your needs:
Here's a couple of sellers on ebay that sell something similar.
http://www.ebay.com/itm/Motor-Mounts-Hemi-5-7-6-1-150-100-/200570011628
http://www.ebay.com/itm/DODGE-5-7L-HEMI-STREET-ROD-MOTOR-MOUNT-KIT-/370450038325
Best of Luck,
Regards,
Joe Dokes