The metal engine mounts are good. You will need different rubber/metal mounts. They will bolt in different holes in the cross member, but the holes are already there. Maybe run a tap through those different holes to clean out the rust.
The 454 is externally balanced, you cannot use the ring gear or flywheel from that engine on the small block. You need all the accessory mounts for the small block, none of the big block parts will work. The accessories them self will work, alternator, P/S pump and maybe the A/C compressor. That depends on the different years of the doner engine that you get the accessory mounts from.
Hopefully the 307 doner is fully dressed with all the accessories, including the A/C compressor if you have AC in the truck. Last problem is the year of the 307, early 307's with the short water pump have different accessory drives and all the accessories sit farther out from the engine.
