Here is the mount for the Marder:
And here is the mount you are refering to:
You can not turn the clutch house to use this mount, then you can't fit the gear carrier properly. I am using the [link=http://www.rc-car-online-shop.de/cgi-bin/cosmoshop/lshop.cgi?action=showdetail&wkid=9427&ls=e&nc=1284564567-9490&rubnum=&artnum=fg_6486&file=&gesamt_zeilen=0Tsuche6486]6486[/link] in my Marder. But i think you can use it if you cut off the upper bar, then you will have room for the clutch housing. You see the lower bar is a bit beefier on the 6486, but i think it will hold up.
But to be sure i would have bought the 6486. How much is he asking for it?
Edit; I don't think it fit! It has not got the thick part on the front of the mount, look at the pictures and compare on [link=http://cgi.ebay.it/09029-Alu-Motorbock-Fg-Sportsline-Zenoah-1-5-7486-NEU-/160468828669?pt=RC_Modellbau&hash=item255cb001fd]this item[/link] on eBay. The upper mount looks taller aswell, where you mount the brace to the differential bearing blocks. Or maybe the bearing sits lower on the 7486.