At 6'3 and 270 lbs, I am the size of a WWE wrestler and the resort has evolved quite a bit since I first visited in 2007 for guests of my size. Back in 2007, when I walked into the line at Raging Spirits, I was taken to the exit and asked to try out the seat. There are no sensors or anything: you need to lower the restraint to a certain point where it touch a ruler placed on the size of the car. Since then, they moved to a fixed 1m95 maximum (6'4) height restriction and looks to have gotten a distinct test seat. As I visited a few times since 2014 and due to my distinctive look, I don't even get checked anymore.
The ride car, shot by me in 2007.
The park released this amazing promo video showing the ride original restraints. They were changed to the current more comfortable ones in 2007:
In 2014, Splash Mountain received new lap bars that replaced the ones currently present on Splash Mountain at WDW. To give you a point of reference, think of a narrower Expedition Everest lap bars and in my case, I struggled to get the restraint over my knees. No picture as the loading dock is quite dark and the ride operators don't allow pictures in loading stations and indoor waiting lines.
Tower of Terror feature 3 point seatbelts like your car. The back row is also quite tight as they kept the original 8 seats versus the other TOT who went to a 7 seat across configuration when they installed seatbelts.
Space Mountain and Journey to the Center of the Earth are first row only and the cast members won't take any chances and send you there.