I don't think many here will have owned a Radian, much less in a Volvo. I have a Radian, and with the angle adjuster (sold separately, good for when they have good head control and sit unassisted), I can fit it in my Mini Cooper. From what I can tell, it has an ample back seat, so it should, but the Volvo has advanced air bags so it can't touch the seat in front (angle adjuster will fix this). If you need the infant angle, you'll likely have to do a middle seat install with the seatbelt.
Re: Diono Radian in a Volvo s40?
I don't think many here will have owned a Radian, much less in a Volvo. I have a Radian, and with the angle adjuster (sold separately, good for when they have good head control and sit unassisted), I can fit it in my Mini Cooper. From what I can tell, it has an ample back seat, so it should, but the Volvo has advanced air bags so it can't touch the seat in front (angle adjuster will fix this). If you need the infant angle, you'll likely have to do a middle seat install with the seatbelt.