You can still get OE urethane front bumper covers, but the rear covers are only available as solid fiberglass, or one of the flexible 'glass styles. Tru-Flex seems to be the brand most folks are the happiest with.
If you go with the urethane front cover, it should be painted separately from the rest of the car, as it needs a special flex agent in the paint to help keep the paint from cracking as it flexes.
If you go with the Tru-Flex, or some other 'glass covers, you'll need to mount them on the car before paint, as 99% of them will need some kind of body work/massaging to fit correctly.
When I did the '74, I got a GM original urethane front cover, but had to settle for a solid 'glass rear cover. There was no such thing as Tru-Flex at the time, so the rear cover and body needed quite a bit of work to get it fitted properly. The front, urethane cover fits just as it did from the factory....not very well at all.
The biggest problem you'll run into if you do these yourself is that the fasteners for the covers will most likely twist off when you try to remove them, meaning you'll prolly also need to buy new retainers for the covers. These are available at most of the aftermarket suppliers, and are usually stainless steel, so they won't tend to rust as the originals did.
Replacing the covers is time consuming, but not too terribly difficult.
______________
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"