Here's how I did the crotch straps. I only use them with my track seat so they go with it when it comes out of the car. The seat itself isn't adjustable for and aft so the crotch straps remain correctly adjusted. The seat itself is very securely attached to the chassis with four 3/8 bolts
Dito on spending the time to adjust belts before you go to the track. Drive the car around for at least 30 mins to get a feel for them. Check out your ability to reach everything with them all tightened up. Fire pull, battery off switch, ignition switch, door release (this one is a stretch in my car). Even the peddles may be in a different place when you are all cinched up in your belts.
Get stuff like peddles, shifter, steering wheel, PERFECT before you go to the track.
Whatever you do don't violate the installation parameters provided by the belt manufacturer. Pay close attention to the webbing's angles from the mounting points to the points that they contact your body.