I bought the Supercab truck after 7 years in a regular cab truck due to the baby seat issue.

My son is 22 months old. When he was born we were out of the country, so he rides in a Britax car seat. As you might guess, it is a British contraption that goes from rear facing to forward facing and is supposed to be good through 5 years old. Because of this it is bulky. It doesn't leave a lot of room for his legs.

We also have a much thinner forward-only car seat and it works like a champ in the Supercab. Since it is easier to put him in and out of that one we leave it in my wife's Jeep since she transports him a lot more than I do. When he gets a bit bigger I'll pick up another one of that style and mothball the Britax for the next young'un.

Best thing to do is to take your child seat with you when you go to test drive the truck, strap it in, strap the kid in, imagine doing it repeatedly for years. If it still works for you, go for it.

The Supercab works like a champ for me and I got it for the same reasons you stated. Need child seat room, want a diesel, don't want the length of a crewcab.