I did it 3 years ago. The road was built for the construction of the hydro project in Radisson Qc. It was made very wide for heayy trucks with wide loads It is asphalt all the way. Only 2 lanes but the gravel shoulders are wide. When I did it, the center line (yellow) was pretty much faded so I wouldn't do it at night. There are some frost heaves spots. Just look for the signs with orange markers, the more the markers, the bigger the heave... The speed limit is 100km, but the trucks and locals go faster. I tried with my truck and TC and couldn't...
Just remember that the road from Matagami to Radisson is over 600 km. There is only one fuel stop half up at about 320 km...
There is a municipal CG in Radisson with power and water. There is also a dump station. There are also free(they ask for a donation) camping spots along the road with no service. just a pit toilet.
Radisson is home to the La Grande Hydro electric stations. There are free guided tours. Bus included (it actually drives down and into the turbine room, carved out of the rock) http://www.hydroquebec.com/visit/
Here is a link to a good website http://www.jamesbayroad.com/
Not much up to date but still good info.
I took 8 days to drive from Ottawa to Radisson and back.(1,200 km one way). No muskeg to drive in, but make sure to choose the right time of year. I went in late August. any earlier and the black flies are out..
Short answer... yes you can drive a car up on the James bay Rd.
Charles