Check the other posts in this section. I have seen where both the mounts were used. I am planning on using a right and a left motor mount on my conversion. If I need to, I will get another left mount, but they are typicaly sold in pairs. Just be sure to get both the engine and frame portions of the mount. Also, it is my understanding that the 351w and the 302w should have the same mount location (the 82 Mustang gt I got my engine from had been swaped to a 351 prior to me purchasing the car.) . The differnce in the engines is higher up, the mounts are in the same location. Good luck on the swap.
Dont forget abut the rear end. The MII was a direct swap for me.
A FAQ for a v8 swap could be helpful. Just be sure to include info on the E-bay manual
Hope this helps