Yup, i like Steves idea. You may have to pump the rafter size up 50mm and cost a little extra but you will make that back in not having to do extra fixings
It seems this link might have gone dead but i just found a site with some DXF files for the same design mounts.
http://wilbo666.pbworks.com/w/browse/#view=ViewFolder¶m=M%20Series%20Engine%20to%20xJZ%20Series%20Engine%20Conversion%20Engine%20Mounts