did the 1973 chevy malibu/elcamino have the heavy duty bumpers like 1974 + with all steel backing or did it just have some very minor steel backing but the same outer skin shape as the 73? any help or pictures would be greatly appreciated.

The '73 Chevelle bumpers were backed with a hefty rebar like the '74s and met the 5 mph impact standards, but for '73 the rear bumpers only met the 2.5 mph impact standard during that transition year. By '74 the rear bumpers met the 5 mph impact standard.

Those big bumpers can rust from the inside out in the area sheilded by the rebar.

Here's a few scans from the '73 Chevelle brochure that are strangely relevent (remember that the big bumpers were introduced that year so GM touted the benefits of these newly mandated behemoths):

I should mention that although the larger cars like the GM intermediates (Chevelles etc.) had reinforcing behind the bumpers, some lighter cars (especially imports) squeaked through the 1973 bumper standards with minor reinforcing and subtle extending of the bumpers out from the body. 1974 will be remembered as the year that the big bumpers arrived on all cars, plus the combination seat / shoulder belts with inertia reels (not a bad thing), the seatbelt / ignition interlock (horrible thing, later repealed).