I know Truck Shop
http://www.truckandcarshop.com/
has a cross member, that already has the engine mounts for a SBC, and just welds to the frame.
If you look on their catalog, it's on page 70.
I'm sure other places have it too. But at $46, it'll save you a few headaches, not having to fab your own motor mounts.
Hope that helps.