Might want to consult with DreamingofSpeed about carseats, she knows her stuff
If anything will justify using the svx for transporting your kids, its the fact that its safe. Hell, your surrounded by a built in roll cage of sorts
Rear legroom isnt bad, as long as the driver doesnt do the ghetto lean...but il refrain from a rant on that and state that I spent 14 hours in the back seat of an svx (7 of which accompinied by a driveshaft and some axles) and it wasnt bad. Its not so much that theres alot of legroom as much as it is the seats are comfortable. Headroom is a bit too little in the back, but otherwise it isnt bad
Oh, and swapping the power belts with manual belts is really easy...you just need the belts and the a pillar trim to do it