I don't know about the Magnum Engine stuff, but when I changed a 318 LA to a 360 LA, the engines' motor mount bosses were about 1/2" different on one side (I think the drivers side?)
Also the 318 lower engine to trans bracket was about 1/2" shorter too. As I recall I just used a big nut as a spacer and longer bracket to trans bolt.
On the engine mount, I think I may have had one mount (think passenger side?) mounted front to back between the frame and motor mount?
That was back around 1980 on my 1973 Power Wagon W150.