High and low beam headlights are different, and the plugs on the back are also different, if I recall correctly, so you can't swap them around like that. I just replaced all four of mine with Sylvania halogens from O'Reilly auto parts, and it made a HUGE difference in my night visibility, especially with the high beams on.
The problem is that the plugs on the new headlamps are not long enough to fully engage the old connectors, so they come off easily. I tried to solve this by putting clear silicone RTV (gasket sealant) on the plugs before installing them. Worked fine at first, but now one of my high beams is loose again and doesn't always work. I don't know of any manufacturer that makes halogens with the correct plug interface. Maybe call one of the Pontiac specialists and see if the headlights they offer have the right connectors.
'68 GTO, 400/TH400, AC/PS/PB/PW, hideaways, Hurst dual gate
'72 442 'Vert, 350/TH350, AC/PS/PB/PW
'74 Corvette, 383/4-speed, 450 hp, restomod
'71 Javelin, 304/3-speed
'70 Cougar XR7 'Vert, 351W/FMX auto *SOLD*