Do whatever is right for your situation. If you end up moving the older child, do it now. Then in a couple weeks put in the baby's carseat so he can get used to it before baby arrives.
Personally, I'd leave the kiddo where he is. Put the baby behind the driver. When it comes time to get in/out of the car you can just open the door for the 3 year old then walk around and put the baby in while he climbs in and sits down. If you do need to stop to check on the baby it's very quick to jump out, open the door behind the driver and do whatever instead of walking all the way around the car.
Also think carpool situations... in our case, it worked to have the older child behind the passenger because when we pull up to the preschool to do drop-off that's the curb-side. It was easy for the teachers to open the door and get the child out and they didn't have to walk all the way around the car.
I wouldn't put the kids next to each other - that's just asking for poking and stealing toys. :-)
Just remember that it's not set in stone. Put the baby behind you and see how it works out. If you don't like it, switch them. If that's worse, switch them back.
My oldest is also very structured (Asperger kids usually are) and hates change with a passion. However, it's actually good to throw them a loop now by making a change so they learn to deal with it. You can make the whole world revolve around them forever so they need to know that everyday life things cause change and they need exposure to it.