I'd be concerned about the torque a 351 will generate compared to the original 260 those mounts were designed to support.
Keep in mind that Ford did not consider them acceptable for use with the 289 HiPo, which got a different set of mounts.
For those who remember the CJ Pony Parts 1966 Shelby Mustang in their showroom, the car has been removed from CJ's property and has entered the restoration phase at Peterson Restorations. He plans to do a YouTube progress series on the car from start to finish...