Some one will help but more info is needed; What are you planning on doing with it, drag, street, 4x4 trail or extreme? Are you planning a cage, different rear? How much time are you planning on?
Edit; welcome to the forum!
Wouldn't "boxing" the frame be a personal preference? The frame without it should be able to handle even the 351 without any detrimental effects. Had to cut a part of mine to clear the headers on the right side. I did weld in the motor mount block on the right side to the cross member to make sure it wouldn't go anywhere.