older child next to evenflo titan sureride 65?

I'm considering this seat for a center lap-belt-only spot in a small car ('93 Ford Escort), so that DD (8 y.o., torso height 18") can stay harnessed there for a year or two longer. Her older brothers (10 y.o. who still needs belt-positioning booster, and 13 y.o. in seatbelt only) also need to sit back there.

Looking at the Sureride at BRU yesterday, I was struck by how relatively deep the back of the shell looks -- i.e. the wider parts of the carseat (the parts that cause trouble in a 3-across install) look like they would end up well forward of the vehicle seat back. In other words, it looked as thought the hips and shoulders of a person sitting next to the Sureride might 'puzzle' well with the seat. Does it work out that way in practice?

At the moment there is a Radian in the middle spot -- DD is now over the top slots (I know the manufacturer says this is OK for the Radian), and DS1's shoulders are getting cramped sitting next to the Radian. DS2 still has little kid shoulders, so it's not an issue for him yet... The Radian is also approaching its expiration date, so I'm considering the Evenflo seat as a replacement.

Any comments, or experience with an older child or adult sitting next to the Sureride, would be much appreciated!

Yes, your older 2 are likely to have a lot more shoulder/hip/buckling space next to a FF Sureride than a Radian.

If she outgrows the Sureride before you're ready for a new car, the one other option is a tethered Ride Safer Travel Vest 2.