This has nothing to do with boxing the frame.....which I wouldn't bother with anway but, have you done some research on the extensive modifications needed to install a 632 in you car?

The fronnt crossmember needs to be notched for pan clearance because of the stroke. And Steering modifications or switching to a rack setup will be needed to clear the pan also.
Also plan on spending at least 1200-1500 for headers to fit in the car.

Just wanted to give you a heads up if you haven't researched it too much.