I am doing an engine swap from 350 to a 454 in my 72 El Camino. I have heard that the frame (engine crossmember) brackets that hold the motor mounts are different for a big block. Is this true? Or will the small block brackets work?

I am still in the process of converting my chevelle from a small block to a big block. Mine isnt fitting right yet but it appears that my exhaust manifolds are incorrect. Anyway The frame mounts should work as long as there are not from vehicle that came with a 307 originally. Just make sure YOU USE SMALL BLOCK ENGINE MOUNTS and they should fit fine. Just test fit the engine mounts to the frame mount before you struggle getting ot to fit like I did. Good Luck
Enjoy the Big Block

68-72 frame mounts are different than 67 mounts. As far as I can tell by pictures I don't think you'll have problem.