My car doesn't have the latch system, but they installed the base for me just fine anyway. After they showed me how I did it myself too and it's not that difficult. Also, the seatbelt is pretty safe. The car seat isn't going anywhere.
I didn't think it's too early to get it installed because the baby could come early. Not only that, but maybe the few weeks before the baby is due you won't want to be dealing with things like that.