[Berg]

As far as I know, the difference between the 307 mounts and the 350,400,396,454 etc style mounts are the height. The 307 uses the taller style frame brackets while the rubber cushion that bolts to the engine is shallow and non-interlocking. The HD mounts use a shorter frame bracket and thicker rubber. If you are going to be putting some serious power down you could convert to the 350-up mounts. otherwise, you will be fine as long as you keep the components matched. When assembled together they are both the same height. If you are worried about the engine comming loose and don't have the money for the new mounts you could always make a limiting strap for the engine. I've seen some factory style limiting straps too. They are in case the engine mount rips since there is nothing to retain it.