I had a '32 Chevrolet coupe that I put in a very strong 265, about 400 H.P. I didn't box the frame (at first) and it would pull the left front tire about 6" off the ground when I left hard. After a few months it cracked the frame clear through. I got another frame and fully boxed it, no more lifting the left whell and after about 10 years no cracking or any problem. Definitely box the frame, but be careful to not concentrate heat from welding in one spot too long, you will put a twist in it that you can't get out. Put the frame on stands to keep it square and level and check it often for any changes while welding. You will never regret boxing it.
