Since the 56 was already set up for a V8 I'm not sure why its a problem...but I've never owned a 56 so I wouldn't know.
I have put a V8 in my 52 panel and the motor mounts that fit in the chassis rails are neat. I do think they make them for the 56 as well but not sure. Here you can see how mine mounted. They come with instrutions and I bolted them on instead of welding.
Don't know if these pics will help or not but take a look.
http://i140.photobucket.com/albums/r19/LutherParris/52Panel/100_0424.jpghttp://i140.photobucket.com/albums/r19/LutherParris/52Panel/100_0430.jpghttp://i140.photobucket.com/albums/r19/LutherParris/52Panel/100_0428.jpgI got these from JimCarter but they actually come from CPP
Correction: They came from Chassis Engineering,sorry for any confusion.Note? if you put the stangII frontend under it I suppose you have a crossmember welded in???? If you do you will probably need to set the engine in the compartment and then weld some bought or home made brackets/mounts to the crossmember then bolt the engine to them. To determine best approach we need to know a little more about what you have done.