hey dan - I've just taken some out of my van as going for some plush colour coded ones - word of note, I had a 3 point belt for the buddy seat in my westy behind the drivers seat, this was v.tight and only just got a car seat to fit - may be worth considering a seat belt extender in amongst your plans or measuring the length of belt required
Mine are surplus to requirements so if you are near wiltshire I've got a 3 point static belt going free