I have a 455 Pontiac in my 84 GP and I am using the 301 motor mounts also but have no clearance problems. I bet you have the mounts in the wrong holes, move the mounts around towards the back of the engine bay until the mounting holes line up. For years I had the mounts in the wrong holes too, the engine sat up too high and too much forward which made me have to use a 15" inch flex fan. It wasn' t until years later that I had to change the engine mounts that I found a better location for them a little back further. The motor dropped down and back a little so then I was able to use a 17" flex fan and the motor sat in the bay perfectly! You can bolt the mount in many different locations due to the many different holes there are on the frame due to several different motors being available for our cars back in the day. There were V8's V6's and even diesel motors that were optional. If I remember correctly I was only able to use 3 out of the 4 holes for the mounts and the last hole was towards the back were there was no hole lined up for the bolt, but that doesn't matter, I've been running it this way for years and the mounts are solid!! So move the mounts back further until the 3 holes line up and you can tack weld the other corner if you'd like but like I said mine have never budged even with the monster torque of the 455! Good Luck!