Bolting or weldintg anything to the upper and lower C channels of the frame will promote stress cracks. If you box the area, the best choice is to use the elliptical shape to avoid 90 degree angles that can concentrate stress points. It's just as easy to do it right the first time than to wish you hadn't taken a shortcut initially. A broken or cracked frame once the vehicle is on the road is really difficult to deal with, and that Caddy is a torque monster. Don't stack the deck against yourself from the get-go. My 1970 Fleetwood Brougham with a 472 engine twisted the center bolt circle out of two flywheels while towing heavy trailers, but it was still running well at 300K miles when the front end wore out so badly we couldn't keep tires on it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!