With my first, all I had is my 86 xtracab. no seats in the back. when I got my 240sx, having to put kids in the back that cannot climb in for themselves is horrible on your back.
I do have a family vehicle now (the Outback) so it's not an issue, but put the kid in the front. Save yourself the pain and aggravation of having to but the kid and seat in the back.
BTW, for the base/carrier units, I don't think it matters if you have a shoulder strap or not. When the booster seat comes into play, then you'll have to worry about it.
Just my thoughts.

Todd

1986 Pickup extracab longbed "Woody edition"
SAS - 33's, 5.29's, lockright rear.

02 IS300 SportCross - my "DD"
94 2wd v6 extracab
04 Outback Wagon- wife's/family car